metrics: Initial integration of Prometheus metrics (#3709)

Signed-off-by: Dave Henderson <dhenderson@gmail.com>

12 files changed, 518 insertions, 33 deletions

diff --git a/admin.go b/admin.go
index acf737e..4ce2099 100644
--- a/admin.go
+++ b/admin.go
@@ -35,6 +35,8 @@ import (
 	"sync"
 	"time"
 
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
 	"go.uber.org/zap"
 )
 
@@ -107,34 +109,53 @@ func (admin AdminConfig) newAdminHandler(addr NetworkAddress) adminHandler {
 		mux:            http.NewServeMux(),
 	}
 
+	addRouteWithMetrics := func(pattern string, handlerLabel string, h http.Handler) {
+		labels := prometheus.Labels{"path": pattern, "handler": handlerLabel}
+		h = promhttp.InstrumentHandlerCounter(
+			adminMetrics.requestCount.MustCurryWith(labels),
+			h,
+		)
+		muxWrap.mux.Handle(pattern, h)
+	}
 	// addRoute just calls muxWrap.mux.Handle after
 	// wrapping the handler with error handling
-	addRoute := func(pattern string, h AdminHandler) {
+	addRoute := func(pattern string, handlerLabel string, h AdminHandler) {
 		wrapper := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 			err := h.ServeHTTP(w, r)
+			if err != nil {
+				labels := prometheus.Labels{
+					"path":    pattern,
+					"handler": handlerLabel,
+					"method":  r.Method,
+				}
+				adminMetrics.requestErrors.With(labels).Inc()
+			}
 			muxWrap.handleError(w, r, err)
 		})
-		muxWrap.mux.Handle(pattern, wrapper)
+		addRouteWithMetrics(pattern, handlerLabel, wrapper)
 	}
 
+	const handlerLabel = "admin"
+
 	// register standard config control endpoints
-	addRoute("/"+rawConfigKey+"/", AdminHandlerFunc(handleConfig))
-	addRoute("/id/", AdminHandlerFunc(handleConfigID))
-	addRoute("/stop", AdminHandlerFunc(handleStop))
+	addRoute("/"+rawConfigKey+"/", handlerLabel, AdminHandlerFunc(handleConfig))
+	addRoute("/id/", handlerLabel, AdminHandlerFunc(handleConfigID))
+	addRoute("/stop", handlerLabel, AdminHandlerFunc(handleStop))
 
 	// register debugging endpoints
-	muxWrap.mux.HandleFunc("/debug/pprof/", pprof.Index)
-	muxWrap.mux.HandleFunc("/debug/pprof/cmdline", pprof.Cmdline)
-	muxWrap.mux.HandleFunc("/debug/pprof/profile", pprof.Profile)
-	muxWrap.mux.HandleFunc("/debug/pprof/symbol", pprof.Symbol)
-	muxWrap.mux.HandleFunc("/debug/pprof/trace", pprof.Trace)
-	muxWrap.mux.Handle("/debug/vars", expvar.Handler())
+	addRouteWithMetrics("/debug/pprof/", handlerLabel, http.HandlerFunc(pprof.Index))
+	addRouteWithMetrics("/debug/pprof/cmdline", handlerLabel, http.HandlerFunc(pprof.Cmdline))
+	addRouteWithMetrics("/debug/pprof/profile", handlerLabel, http.HandlerFunc(pprof.Profile))
+	addRouteWithMetrics("/debug/pprof/symbol", handlerLabel, http.HandlerFunc(pprof.Symbol))
+	addRouteWithMetrics("/debug/pprof/trace", handlerLabel, http.HandlerFunc(pprof.Trace))
+	addRouteWithMetrics("/debug/vars", handlerLabel, expvar.Handler())
 
 	// register third-party module endpoints
 	for _, m := range GetModules("admin.api") {
 		router := m.New().(AdminRouter)
+		handlerLabel := m.ID.Name()
 		for _, route := range router.Routes() {
-			addRoute(route.Pattern, route.Handler)
+			addRoute(route.Pattern, handlerLabel, route.Handler)
 		}
 	}
 
diff --git a/caddyconfig/httpcaddyfile/directives.go b/caddyconfig/httpcaddyfile/directives.go
index d6af97d..9e17eeb 100644
--- a/caddyconfig/httpcaddyfile/directives.go
+++ b/caddyconfig/httpcaddyfile/directives.go
@@ -63,6 +63,7 @@ var directiveOrder = []string{
 
 	// handlers that typically respond to requests
 	"respond",
+	"metrics",
 	"reverse_proxy",
 	"php_fastcgi",
 	"file_server",
diff --git a/go.mod b/go.mod
index fa5913a..7f9cea0 100644
--- a/go.mod
+++ b/go.mod
@@ -17,6 +17,7 @@ require (
 	github.com/mholt/acmez v0.1.1-0.20200811184240-dc9c5f05ed1e
 	github.com/naoina/go-stringutil v0.1.0 // indirect
 	github.com/naoina/toml v0.1.1
+	github.com/prometheus/client_golang v1.7.1
 	github.com/smallstep/certificates v0.15.1
 	github.com/smallstep/cli v0.15.0
 	github.com/smallstep/nosql v0.3.0
diff --git a/go.sum b/go.sum
index 381988e..545c2e1 100644
--- a/go.sum
+++ b/go.sum
@@ -74,6 +74,7 @@ github.com/aws/aws-sdk-go v1.30.29 h1:NXNqBS9hjOCpDL8SyCyl38gZX3LLLunKOJc5E7vJ8P
 github.com/aws/aws-sdk-go v1.30.29/go.mod h1:5zCpMtNQVjRREroY7sYe8lOMRSxkhG6MZveU8YkpAk0=
 github.com/beorn7/perks v0.0.0-20180321164747-3a771d992973/go.mod h1:Dwedo/Wpr24TaqPxmxbtue+5NUziq4I4S80YR8gNf3Q=
 github.com/beorn7/perks v1.0.0/go.mod h1:KWe93zE9D1o94FZ5RNwFwVgaQK1VOXiVxmqh+CedLV8=
+github.com/beorn7/perks v1.0.1 h1:VlbKKnNfV8bJzeqoa4cOKqO6bYr3WgKZxO8Z16+hsOM=
 github.com/beorn7/perks v1.0.1/go.mod h1:G2ZrVWU2WbWT9wwq4/hrbKbnv/1ERSJQ0ibhJ6rlkpw=
 github.com/bgentry/speakeasy v0.1.0/go.mod h1:+zsyZBPWlz7T6j88CTgSN5bM796AkVf0kBD4zp0CCIs=
 github.com/bombsimon/wsl/v2 v2.0.0/go.mod h1:mf25kr/SqFEPhhcxW1+7pxzGlW+hIl/hYTKY95VwV8U=
@@ -85,6 +86,7 @@ github.com/census-instrumentation/opencensus-proto v0.2.0/go.mod h1:f6KPmirojxKA
 github.com/census-instrumentation/opencensus-proto v0.2.1/go.mod h1:f6KPmirojxKA12rnyqOA5BBL4O983OfeGPqjHWSTneU=
 github.com/cespare/xxhash v1.1.0 h1:a6HrQnmkObjyL+Gs60czilIUGqrzKutQD6XZog3p+ko=
 github.com/cespare/xxhash v1.1.0/go.mod h1:XrSqR1VqqWfGrhpAt58auRo0WTKS1nRRg3ghfAqPWnc=
+github.com/cespare/xxhash/v2 v2.1.1 h1:6MnRN8NT7+YBpUIWxHtefFZOKTAPgGjpQSxqLNn0+qY=
 github.com/cespare/xxhash/v2 v2.1.1/go.mod h1:VGX0DQ3Q6kWi7AoAeZDth3/j3BFtOZR5XLFGgcrjCOs=
 github.com/cheekybits/genny v1.0.0 h1:uGGa4nei+j20rOSeDeP5Of12XVm7TGUd4dJA9RDitfE=
 github.com/cheekybits/genny v1.0.0/go.mod h1:+tQajlRqAUrPI7DOSpB0XAqZYtQakVtB7wXkRAgjxjQ=
@@ -140,9 +142,7 @@ github.com/fatih/color v1.6.0/go.mod h1:Zm6kSWBoL9eyXnKyktHP6abPY2pDugNf5Kwzbycv
 github.com/fatih/color v1.7.0/go.mod h1:Zm6kSWBoL9eyXnKyktHP6abPY2pDugNf5KwzbycvMj4=
 github.com/fatih/color v1.8.0/go.mod h1:3l45GVGkyrnYNl9HoIjnp2NnNWvh6hLAqD8yTfGjnw8=
 github.com/flynn/go-shlex v0.0.0-20150515145356-3f9db97f8568/go.mod h1:xEzjJPgXI435gkrCt3MPfRiAkVrwSbHsst4LCFVfpJc=
-github.com/francoispqt/gojay v1.2.13 h1:d2m3sFjloqoIUQU3TsHBgj6qg/BVGlTBeHDUmyJnXKk=
 github.com/francoispqt/gojay v1.2.13/go.mod h1:ehT5mTG4ua4581f1++1WLG0vPdaA9HaiDsoyrBGkyDY=
-github.com/fsnotify/fsnotify v1.4.7 h1:IXs+QLmnXW2CcXuY+8Mzv/fWEsPGWxqefPtCP5CnV9I=
 github.com/fsnotify/fsnotify v1.4.7/go.mod h1:jwhsz4b93w/PPRr/qN1Yymfu8t87LnFCMoQvtojpjFo=
 github.com/fsnotify/fsnotify v1.4.9 h1:hsms1Qyu0jgnwNXIxa+/V/PDsU6CfLf6CNO8H7IWoS4=
 github.com/fsnotify/fsnotify v1.4.9/go.mod h1:znqG4EE+3YCdAaPaxE2ZRY/06pZUdp0tY4IgpuI1SZQ=
@@ -196,7 +196,6 @@ github.com/golang/mock v1.0.0/go.mod h1:oTYuIxOrZwtPieC+H1uAHpcLFnEyAGVDL/k47Jfb
 github.com/golang/mock v1.1.1/go.mod h1:oTYuIxOrZwtPieC+H1uAHpcLFnEyAGVDL/k47Jfbm0A=
 github.com/golang/mock v1.2.0/go.mod h1:oTYuIxOrZwtPieC+H1uAHpcLFnEyAGVDL/k47Jfbm0A=
 github.com/golang/mock v1.3.1/go.mod h1:sBzyDLLjw3U8JLTeZvSv8jJB+tU5PVekmnlKIyFUx0Y=
-github.com/golang/mock v1.4.0 h1:Rd1kQnQu0Hq3qvJppYSG0HtP+f5LPPUiDswTLiEegLg=
 github.com/golang/mock v1.4.0/go.mod h1:UOMv5ysSaYNkG+OFQykRIcU/QvvxJf3p21QfJ2Bt3cw=
 github.com/golang/mock v1.4.4 h1:l75CXGRSwbaYNpl/Z2X1XIIAMSCquvXgpVZDhwEIJsc=
 github.com/golang/mock v1.4.4/go.mod h1:l3mdAwkq5BuhzHwde/uurv3sEJeZMXNpwsxVWU71h+4=
@@ -209,9 +208,7 @@ github.com/golang/protobuf v1.4.0-rc.1/go.mod h1:ceaxUfeHdC40wWswd/P6IGgMaK3YpKi
 github.com/golang/protobuf v1.4.0-rc.1.0.20200221234624-67d41d38c208/go.mod h1:xKAWHe0F5eneWXFV3EuXVDTCmh+JuBKY0li0aMyXATA=
 github.com/golang/protobuf v1.4.0-rc.2/go.mod h1:LlEzMj4AhA7rCAGe4KMBDvJI+AwstrUpVNzEA03Pprs=
 github.com/golang/protobuf v1.4.0-rc.4.0.20200313231945-b860323f09d0/go.mod h1:WU3c8KckQ9AFe+yFwt9sWVRKCVIyN9cPHBJSNnbL67w=
-github.com/golang/protobuf v1.4.0 h1:oOuy+ugB+P/kBdUnG5QaMXSIyJ1q38wWSojYCb3z5VQ=
 github.com/golang/protobuf v1.4.0/go.mod h1:jodUvKwWbYaEsadDk5Fwe5c77LiNKVO9IDvqG2KuDX0=
-github.com/golang/protobuf v1.4.1 h1:ZFgWrT+bLgsYPirOnRfKLYJLvssAegOj/hgyMFdJZe0=
 github.com/golang/protobuf v1.4.1/go.mod h1:U8fpvMrcmy5pZrNK1lt4xCsGvpyWQ/VVv6QDs8UjoX8=
 github.com/golang/protobuf v1.4.2 h1:+Z5KGCizgyZCbGh1KZqA0fcLLkwbsjIzS4aV2v7wJX0=
 github.com/golang/protobuf v1.4.2/go.mod h1:oDoupMAO8OvCJWAcko0GGGIgR6R6ocIYbsSw735rRwI=
@@ -291,7 +288,6 @@ github.com/hashicorp/golang-lru v0.5.0/go.mod h1:/m3WP610KZHVQ1SGc6re/UDhFvYD7pJ
 github.com/hashicorp/golang-lru v0.5.1/go.mod h1:/m3WP610KZHVQ1SGc6re/UDhFvYD7pJ4Ao+sR/qLZy8=
 github.com/hashicorp/hcl v0.0.0-20180404174102-ef8a98b0bbce/go.mod h1:oZtUIOe8dh44I2q6ScRibXws4Ajl+d+nod3AaR9vL5w=
 github.com/hashicorp/hcl v1.0.0/go.mod h1:E5yfLk+7swimpb2L/Alb/PJmXilQ/rhwaUYs4T20WEQ=
-github.com/hpcloud/tail v1.0.0 h1:nfCOvKYfkgYP8hkirhJocXT2+zOD8yUNjXaWfTlyFKI=
 github.com/hpcloud/tail v1.0.0/go.mod h1:ab1qPbhIpdTxEkNHXyeSf5vhxWSCs/tWer42PpOxQnU=
 github.com/huandu/xstrings v1.3.1 h1:4jgBlKK6tLKFvO8u5pmYjG91cqytmDCDvGh7ECVFfFs=
 github.com/huandu/xstrings v1.3.1/go.mod h1:y5/lhBue+AyNmUVz9RLU9xbLR0o4KIIExikq4ovT0aE=
@@ -309,6 +305,7 @@ github.com/jonboulle/clockwork v0.1.0/go.mod h1:Ii8DK3G1RaLaWxj9trq07+26W01tbo22
 github.com/json-iterator/go v1.1.6/go.mod h1:+SdeFBvtyEkXs7REEP0seUULqWtbJapLOCVDaaPEHmU=
 github.com/json-iterator/go v1.1.8/go.mod h1:KdQUCv79m/52Kvf8AW2vK1V8akMuk1QjK/uOdHXbAo4=
 github.com/json-iterator/go v1.1.9/go.mod h1:KdQUCv79m/52Kvf8AW2vK1V8akMuk1QjK/uOdHXbAo4=
+github.com/json-iterator/go v1.1.10/go.mod h1:KdQUCv79m/52Kvf8AW2vK1V8akMuk1QjK/uOdHXbAo4=
 github.com/jstemmer/go-junit-report v0.0.0-20190106144839-af01ea7f8024/go.mod h1:6v2b51hI/fHJwM22ozAgKL4VKDeJcHhJFhtBdhmNjmU=
 github.com/jstemmer/go-junit-report v0.9.1/go.mod h1:Brl9GWCQeLvo8nXZwPNNblvFj/XSXhF0NWZEnDohbsk=
 github.com/jsternberg/zap-logfmt v1.2.0 h1:1v+PK4/B48cy8cfQbxL4FmmNZrjnIMr2BsnyEmXqv2o=
@@ -380,6 +377,7 @@ github.com/mattn/go-runewidth v0.0.4/go.mod h1:LwmH8dsx7+W8Uxz3IHJYH5QSwggIsqBzp
 github.com/mattn/go-runewidth v0.0.7/go.mod h1:H031xJmbD/WCDINGzjvQ9THkh0rPKHF+m2gUSrubnMI=
 github.com/mattn/go-sqlite3 v1.10.0/go.mod h1:FPy6KqzDD04eiIsT53CuJW3U88zkxoIYsOqkbpncsNc=
 github.com/mattn/goveralls v0.0.2/go.mod h1:8d1ZMHsd7fW6IRPKQh46F2WRpyib5/X4FOpevwGNQEw=
+github.com/matttproud/golang_protobuf_extensions v1.0.1 h1:4hp9jkHxhMHkqkrB3Ix0jegS5sx/RkqARlsWZ6pIwiU=
 github.com/matttproud/golang_protobuf_extensions v1.0.1/go.mod h1:D8He9yQNgCq6Z5Ld7szi9bcBfOoFv/3dc6xSMkL2PC0=
 github.com/mholt/acmez v0.1.1-0.20200811184240-dc9c5f05ed1e h1:F5E9CM0APrZtTfowJfvUBmrCjuHLfslZyFlNTEB7UV4=
 github.com/mholt/acmez v0.1.1-0.20200811184240-dc9c5f05ed1e/go.mod h1:8qnn8QA/Ewx8E3ZSsmscqsIjhhpxuy9vqdgbX2ceceM=
@@ -425,7 +423,6 @@ github.com/olekukonko/tablewriter v0.0.4/go.mod h1:zq6QwlOf5SlnkVbMSr5EoBv3636FW
 github.com/onsi/ginkgo v0.0.0-20170829012221-11459a886d9c/go.mod h1:lLunBs/Ym6LB5Z9jYTR76FiuTmxDTDusOGeTQH+WWjE=
 github.com/onsi/ginkgo v1.6.0/go.mod h1:lLunBs/Ym6LB5Z9jYTR76FiuTmxDTDusOGeTQH+WWjE=
 github.com/onsi/ginkgo v1.10.1/go.mod h1:lLunBs/Ym6LB5Z9jYTR76FiuTmxDTDusOGeTQH+WWjE=
-github.com/onsi/ginkgo v1.11.0 h1:JAKSXpt1YjtLA7YpPiqO9ss6sNXEsPfSGdwN0UHqzrw=
 github.com/onsi/ginkgo v1.11.0/go.mod h1:lLunBs/Ym6LB5Z9jYTR76FiuTmxDTDusOGeTQH+WWjE=
 github.com/onsi/ginkgo v1.12.1/go.mod h1:zj2OWP4+oCPe1qIXoGWkgMRwljMUYCdkwsT2108oapk=
 github.com/onsi/ginkgo v1.14.0 h1:2mOpI4JVVPBN+WQRa0WKH2eXR+Ey+uK4n7Zj0aYpIQA=
@@ -434,7 +431,6 @@ github.com/onsi/gomega v0.0.0-20170829124025-dcabb60a477c/go.mod h1:C1qb7wdrVGGV
 github.com/onsi/gomega v1.4.2/go.mod h1:ex+gbHU/CVuBBDIJjb2X0qEXbFg53c61hWP/1CpauHY=
 github.com/onsi/gomega v1.7.0/go.mod h1:ex+gbHU/CVuBBDIJjb2X0qEXbFg53c61hWP/1CpauHY=
 github.com/onsi/gomega v1.7.1/go.mod h1:XdKZgCCFLUoM/7CFJVPcG8C1xQ1AJ0vpAezJrB7JYyY=
-github.com/onsi/gomega v1.8.1 h1:C5Dqfs/LeauYDX0jJXIe2SWmwCbGzx9yF8C8xy3Lh34=
 github.com/onsi/gomega v1.8.1/go.mod h1:Ho0h+IUsWyvy1OpqCwxlQ/21gkhVunqlU8fDGcoTdcA=
 github.com/onsi/gomega v1.10.1 h1:o0+MgICZLuZ7xjH7Vx6zS/zcu93/BEp1VwkIW1mEXCE=
 github.com/onsi/gomega v1.10.1/go.mod h1:iN09h71vgCQne3DLsj+A5owkum+a2tYe+TOCB1ybHNo=
@@ -458,20 +454,28 @@ github.com/prometheus/client_golang v0.9.3/go.mod h1:/TN21ttK/J9q6uSwhBd54HahCDf
 github.com/prometheus/client_golang v0.9.4/go.mod h1:oCXIBxdI62A4cR6aTRJCgetEjecSIYzOEaeAn4iYEpM=
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+github.com/prometheus/client_golang v1.7.1 h1:NTGy1Ja9pByO+xAeH/qiWnLrKtr3hJPNjaVUwnjpdpA=
+github.com/prometheus/client_golang v1.7.1/go.mod h1:PY5Wy2awLA44sXw4AOSfFBetzPP4j5+D6mVACh+pe2M=
 github.com/prometheus/client_model v0.0.0-20180712105110-5c3871d89910/go.mod h1:MbSGuTsp3dbXC40dX6PRTWyKYBIrTGTE9sqQNg2J8bo=
 github.com/prometheus/client_model v0.0.0-20190129233127-fd36f4220a90/go.mod h1:xMI15A0UPsDsEKsMN9yxemIoYk6Tm2C1GtYGdfGttqA=
 github.com/prometheus/client_model v0.0.0-20190812154241-14fe0d1b01d4/go.mod h1:xMI15A0UPsDsEKsMN9yxemIoYk6Tm2C1GtYGdfGttqA=
 github.com/prometheus/client_model v0.1.0/go.mod h1:xMI15A0UPsDsEKsMN9yxemIoYk6Tm2C1GtYGdfGttqA=
+github.com/prometheus/client_model v0.2.0 h1:uq5h0d+GuxiXLJLNABMgp2qUWDPiLvgCzz2dUR+/W/M=
+github.com/prometheus/client_model v0.2.0/go.mod h1:xMI15A0UPsDsEKsMN9yxemIoYk6Tm2C1GtYGdfGttqA=
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 github.com/prometheus/common v0.0.0-20181113130724-41aa239b4cce/go.mod h1:daVV7qP5qjZbuso7PdcryaAu0sAZbrN9i7WWcTMWvro=
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+github.com/prometheus/common v0.10.0 h1:RyRA7RzGXQZiW+tGMr7sxa85G1z0yOpM1qq5c8lNawc=
+github.com/prometheus/common v0.10.0/go.mod h1:Tlit/dnDKsSWFlCLTWaA1cyBgKHSMdTB80sz/V91rCo=
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 github.com/prometheus/procfs v0.0.0-20190507164030-5867b95ac084/go.mod h1:TjEm7ze935MbeOT/UhFTIMYKhuLP4wbCsTZCD3I8kEA=
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+github.com/prometheus/procfs v0.1.3 h1:F0+tqvhOksq22sc6iCHF5WGlWjdwj92p0udFh1VFBS8=
+github.com/prometheus/procfs v0.1.3/go.mod h1:lV6e/gmhEcM9IjHGsFOCxxuZ+z1YqCvr4OA4YeYWdaU=
 github.com/prometheus/tsdb v0.7.1/go.mod h1:qhTCs0VvXwvX/y3TZrWD7rabWM+ijKTux40TwIPHuXU=
 github.com/quasilyte/go-consistent v0.0.0-20190521200055-c6f3937de18c/go.mod h1:5STLWrekHfjyYwxBRVRXNOSewLJ3PWfDJd1VyTS21fI=
 github.com/rogpeppe/fastuuid v0.0.0-20150106093220-6724a57986af/go.mod h1:XWv6SoW27p1b0cqNHllgS5HIMJraePCO15w5zCzIWYg=
@@ -527,11 +531,9 @@ github.com/sirupsen/logrus v1.4.1/go.mod h1:ni0Sbl8bgC9z8RoU9G6nDWqqs/fq4eDPysMB
 github.com/sirupsen/logrus v1.4.2 h1:SPIRibHv4MatM3XXNO2BJeFLZwZ2LvZgfQ5+UNI2im4=
 github.com/sirupsen/logrus v1.4.2/go.mod h1:tLMulIdttU9McNUspp0xgXVQah82FyeX6MwdIuYE2rE=
 github.com/smallstep/assert v0.0.0-20180720014142-de77670473b5/go.mod h1:TC9A4+RjIOS+HyTH7wG17/gSqVv95uDw2J64dQZx7RE=
-github.com/smallstep/assert v0.0.0-20200103212524-b99dc1097b15 h1:kSImCuenAkXtCaBeQ1UhmzzJGRhSm8sVH7I3sHE2Qdg=
 github.com/smallstep/assert v0.0.0-20200103212524-b99dc1097b15/go.mod h1:MyOHs9Po2fbM1LHej6sBUT8ozbxmMOFG+E+rx/GSGuc=
 github.com/smallstep/assert v0.0.0-20200723003110-82e2b9b3b262 h1:unQFBIznI+VYD1/1fApl1A+9VcBk+9dcqGfnePY87LY=
 github.com/smallstep/assert v0.0.0-20200723003110-82e2b9b3b262/go.mod h1:MyOHs9Po2fbM1LHej6sBUT8ozbxmMOFG+E+rx/GSGuc=
-github.com/smallstep/certificates v0.15.0 h1:riIt3fBWuenlvHoqkVuUOxQbhtxFQMO1ziSr7usb1J0=
 github.com/smallstep/certificates v0.15.0/go.mod h1:awyVXYIVn4J9ANSSnDuA3FjKia7+QixaqorGW8CKGq8=
 github.com/smallstep/certificates v0.15.1 h1:Es2IGgU4D0RE4OKys/VB1KvAuGuZwkxR7D9A0DeM/Ow=
 github.com/smallstep/certificates v0.15.1/go.mod h1:hDmNejfuVTDDe1NBIKrhL9xjELRJb2ZFM+bDXwSW9xI=
@@ -627,7 +629,6 @@ go.opencensus.io v0.22.0/go.mod h1:+kGneAE2xo2IficOXnaByMWTGM9T73dGwxeWcUqIpI8=
 go.opencensus.io v0.22.2 h1:75k/FF0Q2YM8QYo07VPddOLBslDt1MZOdEslOHvmzAs=
 go.opencensus.io v0.22.2/go.mod h1:yxeiOL68Rb0Xd1ddK5vPZ/oVn4vY4Ynel7k9FzqtOIw=
 go.step.sm/crypto v0.0.0-20200805202904-ec18b6df3cf0/go.mod h1:8VYxmvSKt5yOTBx3MGsD2Gk4F1Es/3FIxrjnfeYWE8U=
-go.step.sm/crypto v0.1.1 h1:xg3kUS30hEnwgbxtKwq9a4MJaeiU616HSug60LU9B2E=
 go.step.sm/crypto v0.1.1/go.mod h1:cIoSWTfTQ5xqvwTeZH9ZXZzi6jdMepjK4A/TDWMUvw8=
 go.step.sm/crypto v0.2.0 h1:Rx+XqrNO4ZGHWlT9QCXls2L9pvcNiI23zEpAq0fctvY=
 go.step.sm/crypto v0.2.0/go.mod h1:YNLnHj4JgABFoRkUq8brkscIB9THdiJUFoDxLQw1tww=
@@ -769,10 +770,10 @@ golang.org/x/sys v0.0.0-20200116001909-b77594299b42/go.mod h1:h1NjWce9XRLGQEsW7w
 golang.org/x/sys v0.0.0-20200223170610-d5e6a3e2c0ae/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20200302150141-5c8b2ff67527/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20200323222414-85ca7c5b95cd/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
-golang.org/x/sys v0.0.0-20200413165638-669c56c373c4 h1:opSr2sbRXk5X5/givKrrKj9HXxFpW2sdCiP8MJSKLQY=
 golang.org/x/sys v0.0.0-20200413165638-669c56c373c4/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
-golang.org/x/sys v0.0.0-20200519105757-fe76b779f299 h1:DYfZAGf2WMFjMxbgTjaC+2HC7NkNAQs+6Q8b9WEB/F4=
 golang.org/x/sys v0.0.0-20200519105757-fe76b779f299/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
+golang.org/x/sys v0.0.0-20200615200032-f1bc736245b1 h1:ogLJMz+qpzav7lGMh10LMvAkM/fAoGlaiiHYiFYdm80=
+golang.org/x/sys v0.0.0-20200615200032-f1bc736245b1/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
 golang.org/x/text v0.0.0-20170915090833-1cbadb444a80/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
 golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
@@ -864,9 +865,7 @@ google.golang.org/genproto v0.0.0-20190819201941-24fa4b261c55/go.mod h1:DMBHOl98
 google.golang.org/genproto v0.0.0-20190911173649-1774047e7e51/go.mod h1:IbNlFCBrqXvoKpeg0TB2l7cyZUmoaFKYIwrEpbDKLA8=
 google.golang.org/genproto v0.0.0-20190927181202-20e1ac93f88c/go.mod h1:IbNlFCBrqXvoKpeg0TB2l7cyZUmoaFKYIwrEpbDKLA8=
 google.golang.org/genproto v0.0.0-20191230161307-f3c370f40bfb/go.mod h1:n3cpQtvxv34hfy77yVDNjmbRyujviMdxYliBSkLhpCc=
-google.golang.org/genproto v0.0.0-20200305110556-506484158171 h1:xes2Q2k+d/+YNXVw0FpZkIDJiaux4OVrRKXRAzH6A0U=
 google.golang.org/genproto v0.0.0-20200305110556-506484158171/go.mod h1:55QSHmfGQM9UVYDPBsyGGes0y52j32PQ3BqQfXhyH3c=
-google.golang.org/genproto v0.0.0-20200526211855-cb27e3aa2013 h1:+kGHl1aib/qcwaRi1CbqBZ1rk19r85MNUf8HaBghugY=
 google.golang.org/genproto v0.0.0-20200526211855-cb27e3aa2013/go.mod h1:NbSheEEYHJ7i3ixzK3sjbqSGDJWnxyFXZblF3eUsNvo=
 google.golang.org/genproto v0.0.0-20200806141610-86f49bd18e98 h1:LCO0fg4kb6WwkXQXRQQgUYsFeFb5taTX5WAx5O/Vt28=
 google.golang.org/genproto v0.0.0-20200806141610-86f49bd18e98/go.mod h1:FWY/as6DDZQgahTzZj3fqbO1CbirC29ZNUFHwi0/+no=
@@ -889,9 +888,7 @@ google.golang.org/protobuf v0.0.0-20200228230310-ab0ca4ff8a60/go.mod h1:cfTl7dwQ
 google.golang.org/protobuf v1.20.1-0.20200309200217-e05f789c0967/go.mod h1:A+miEFZTKqfCUM6K7xSMQL9OKL/b6hQv+e19PK+JZNE=
 google.golang.org/protobuf v1.21.0/go.mod h1:47Nbq4nVaFHyn7ilMalzfO3qCViNmqZ2kzikPIcrTAo=
 google.golang.org/protobuf v1.22.0/go.mod h1:EGpADcykh3NcUnDUJcl1+ZksZNG86OlYog2l/sGQquU=
-google.golang.org/protobuf v1.23.0 h1:4MY060fB1DLGMB/7MBTLnwQUY6+F09GEiz6SsrNqyzM=
 google.golang.org/protobuf v1.23.0/go.mod h1:EGpADcykh3NcUnDUJcl1+ZksZNG86OlYog2l/sGQquU=
-google.golang.org/protobuf v1.23.1-0.20200526195155-81db48ad09cc h1:TnonUr8u3himcMY0vSh23jFOXA+cnucl1gB6EQTReBI=
 google.golang.org/protobuf v1.23.1-0.20200526195155-81db48ad09cc/go.mod h1:EGpADcykh3NcUnDUJcl1+ZksZNG86OlYog2l/sGQquU=
 google.golang.org/protobuf v1.24.0 h1:UhZDfRO8JRQru4/+LlLE0BRKGF8L+PICnvYZmx/fEGA=
 google.golang.org/protobuf v1.24.0/go.mod h1:r/3tXBNzIEhYS9I1OUVjXDlt8tc493IdKGjtUeSXeh4=
@@ -903,7 +900,6 @@ gopkg.in/check.v1 v1.0.0-20190902080502-41f04d3bba15 h1:YR8cESwS4TdDjEe65xsg0ogR
 gopkg.in/check.v1 v1.0.0-20190902080502-41f04d3bba15/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/cheggaaa/pb.v1 v1.0.28/go.mod h1:V/YB90LKu/1FcN3WVnfiiE5oMCibMjukxqG/qStrOgw=
 gopkg.in/errgo.v2 v2.1.0/go.mod h1:hNsd1EY+bozCKY1Ytp96fpM3vjJbqLJn88ws8XvfDNI=
-gopkg.in/fsnotify.v1 v1.4.7 h1:xOHLXZwVvI9hhs+cLKq5+I5onOuwQLhQwiu63xxlHs4=
 gopkg.in/fsnotify.v1 v1.4.7/go.mod h1:Tz8NjZHkW78fSQdbUxIjBTcgA1z1m8ZHf0WmKUhAMys=
 gopkg.in/gemnasium/logrus-airbrake-hook.v2 v2.1.2/go.mod h1:Xk6kEKp8OKb+X14hQBKWaSkCsqBpgog8nAV2xsGOxlo=
 gopkg.in/inf.v0 v0.9.1/go.mod h1:cWUDdTG/fYaXco+Dcufb5Vnc6Gp2YChqWtbxRZE0mXw=
diff --git a/metrics.go b/metrics.go
new file mode 100644
index 0000000..a7ff3b7
--- /dev/null
+++ b/metrics.go
@@ -0,0 +1,32 @@
+package caddy
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+)
+
+// define and register the metrics used in this package.
+func init() {
+	prometheus.MustRegister(prometheus.NewBuildInfoCollector())
+
+	const ns, sub = "caddy", "admin"
+
+	adminMetrics.requestCount = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: ns,
+		Subsystem: sub,
+		Name:      "http_requests_total",
+		Help:      "Counter of requests made to the Admin API's HTTP endpoints.",
+	}, []string{"handler", "path", "code", "method"})
+	adminMetrics.requestErrors = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: ns,
+		Subsystem: sub,
+		Name:      "http_request_errors_total",
+		Help:      "Number of requests resulting in middleware errors.",
+	}, []string{"handler", "path", "method"})
+}
+
+// adminMetrics is a collection of metrics that can be tracked for the admin API.
+var adminMetrics = struct {
+	requestCount  *prometheus.CounterVec
+	requestErrors *prometheus.CounterVec
+}{}
diff --git a/modules/caddyhttp/app.go b/modules/caddyhttp/app.go
index 375ca4d..c8bbc95 100644
--- a/modules/caddyhttp/app.go
+++ b/modules/caddyhttp/app.go
@@ -226,6 +226,8 @@ func (app *App) Provision(ctx caddy.Context) error {
 		// route handler so that important security checks are done, etc.
 		primaryRoute := emptyHandler
 		if srv.Routes != nil {
+			// inject the server name for observability purposes
+			ctx.Context = contextWithServerName(ctx.Context, srvName)
 			err := srv.Routes.ProvisionHandlers(ctx)
 			if err != nil {
 				return fmt.Errorf("server %s: setting up route handlers: %v", srvName, err)
diff --git a/modules/caddyhttp/metrics.go b/modules/caddyhttp/metrics.go
new file mode 100644
index 0000000..dbf1033
--- /dev/null
+++ b/modules/caddyhttp/metrics.go
@@ -0,0 +1,195 @@
+package caddyhttp
+
+import (
+	"context"
+	"net/http"
+	"strconv"
+	"sync"
+	"time"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+)
+
+var httpMetrics = struct {
+	init             sync.Once
+	requestInFlight  *prometheus.GaugeVec
+	requestCount     *prometheus.CounterVec
+	requestErrors    *prometheus.CounterVec
+	requestDuration  *prometheus.HistogramVec
+	requestSize      *prometheus.HistogramVec
+	responseSize     *prometheus.HistogramVec
+	responseDuration *prometheus.HistogramVec
+}{
+	init: sync.Once{},
+}
+
+func initHTTPMetrics() {
+	const ns, sub = "caddy", "http"
+
+	basicLabels := []string{"server", "handler"}
+	httpMetrics.requestInFlight = promauto.NewGaugeVec(prometheus.GaugeOpts{
+		Namespace: ns,
+		Subsystem: sub,
+		Name:      "requests_in_flight",
+		Help:      "Number of requests currently handled by this server.",
+	}, basicLabels)
+	httpMetrics.requestErrors = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: ns,
+		Subsystem: sub,
+		Name:      "request_errors_total",
+		Help:      "Number of requests resulting in middleware errors.",
+	}, basicLabels)
+	httpMetrics.requestCount = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: ns,
+		Subsystem: sub,
+		Name:      "requests_total",
+		Help:      "Counter of HTTP(S) requests made.",
+	}, basicLabels)
+
+	// TODO: allow these to be customized in the config
+	durationBuckets := prometheus.DefBuckets
+	sizeBuckets := prometheus.ExponentialBuckets(256, 4, 8)
+
+	httpLabels := []string{"server", "handler", "code", "method"}
+	httpMetrics.requestDuration = promauto.NewHistogramVec(prometheus.HistogramOpts{
+		Namespace: ns,
+		Subsystem: sub,
+		Name:      "request_duration_seconds",
+		Help:      "Histogram of round-trip request durations.",
+		Buckets:   durationBuckets,
+	}, httpLabels)
+	httpMetrics.requestSize = promauto.NewHistogramVec(prometheus.HistogramOpts{
+		Namespace: ns,
+		Subsystem: sub,
+		Name:      "request_size_bytes",
+		Help:      "Total size of the request. Includes body",
+		Buckets:   sizeBuckets,
+	}, httpLabels)
+	httpMetrics.responseSize = promauto.NewHistogramVec(prometheus.HistogramOpts{
+		Namespace: ns,
+		Subsystem: sub,
+		Name:      "response_size_bytes",
+		Help:      "Size of the returned response.",
+		Buckets:   sizeBuckets,
+	}, httpLabels)
+	httpMetrics.responseDuration = promauto.NewHistogramVec(prometheus.HistogramOpts{
+		Namespace: ns,
+		Subsystem: sub,
+		Name:      "response_duration_seconds",
+		Help:      "Histogram of times to first byte in response bodies.",
+		Buckets:   durationBuckets,
+	}, httpLabels)
+}
+
+type ctxKeyServerName struct{}
+
+// serverNameFromContext extracts the current server name from the context.
+// Returns "UNKNOWN" if none is available (should probably never happen?)
+func serverNameFromContext(ctx context.Context) string {
+	srvName, ok := ctx.Value(ctxKeyServerName{}).(string)
+	if !ok {
+		return "UNKNOWN"
+	}
+	return srvName
+}
+
+func contextWithServerName(ctx context.Context, serverName string) context.Context {
+	return context.WithValue(ctx, ctxKeyServerName{}, serverName)
+}
+
+type metricsInstrumentedHandler struct {
+	labels       prometheus.Labels
+	statusLabels prometheus.Labels
+	mh           MiddlewareHandler
+}
+
+func newMetricsInstrumentedHandler(server, handler string, mh MiddlewareHandler) *metricsInstrumentedHandler {
+	httpMetrics.init.Do(func() {
+		initHTTPMetrics()
+	})
+
+	labels := prometheus.Labels{"server": server, "handler": handler}
+	statusLabels := prometheus.Labels{"server": server, "handler": handler, "code": "", "method": ""}
+	return &metricsInstrumentedHandler{labels, statusLabels, mh}
+}
+
+func (h *metricsInstrumentedHandler) ServeHTTP(w http.ResponseWriter, r *http.Request, next Handler) error {
+	inFlight := httpMetrics.requestInFlight.With(h.labels)
+	inFlight.Inc()
+	defer inFlight.Dec()
+
+	statusLabels := prometheus.Labels{"method": r.Method}
+	for k, v := range h.labels {
+		statusLabels[k] = v
+	}
+
+	start := time.Now()
+
+	// This is a _bit_ of a hack - it depends on the ShouldBufferFunc always
+	// being called when the headers are written.
+	// Effectively the same behaviour as promhttp.InstrumentHandlerTimeToWriteHeader.
+	writeHeaderRecorder := ShouldBufferFunc(func(status int, header http.Header) bool {
+		statusLabels["code"] = sanitizeCode(status)
+		ttfb := time.Since(start).Seconds()
+		observeWithExemplar(statusLabels, httpMetrics.responseDuration, ttfb)
+		return false
+	})
+	wrec := NewResponseRecorder(w, nil, writeHeaderRecorder)
+	err := h.mh.ServeHTTP(wrec, r, next)
+	dur := time.Since(start).Seconds()
+	httpMetrics.requestCount.With(h.labels).Inc()
+	if err != nil {
+		httpMetrics.requestErrors.With(h.labels).Inc()
+		return err
+	}
+
+	observeWithExemplar(statusLabels, httpMetrics.requestDuration, dur)
+	observeWithExemplar(statusLabels, httpMetrics.requestSize, float64(computeApproximateRequestSize(r)))
+	httpMetrics.responseSize.With(statusLabels).Observe(float64(wrec.Size()))
+
+	return nil
+}
+
+func observeWithExemplar(l prometheus.Labels, o *prometheus.HistogramVec, value float64) {
+	obs := o.With(l)
+	if oe, ok := obs.(prometheus.ExemplarObserver); ok {
+		oe.ObserveWithExemplar(value, l)
+		return
+	}
+	// _should_ be a noop, but here just in case...
+	obs.Observe(value)
+}
+
+func sanitizeCode(code int) string {
+	if code == 0 {
+		return "200"
+	}
+	return strconv.Itoa(code)
+
+}
+
+// taken from https://github.com/prometheus/client_golang/blob/6007b2b5cae01203111de55f753e76d8dac1f529/prometheus/promhttp/instrument_server.go#L298
+func computeApproximateRequestSize(r *http.Request) int {
+	s := 0
+	if r.URL != nil {
+		s += len(r.URL.String())
+	}
+
+	s += len(r.Method)
+	s += len(r.Proto)
+	for name, values := range r.Header {
+		s += len(name)
+		for _, value := range values {
+			s += len(value)
+		}
+	}
+	s += len(r.Host)
+
+	// N.B. r.Form and r.MultipartForm are assumed to be included in r.URL.
+
+	if r.ContentLength != -1 {
+		s += int(r.ContentLength)
+	}
+	return s
+}
diff --git a/modules/caddyhttp/metrics_test.go b/modules/caddyhttp/metrics_test.go
new file mode 100644
index 0000000..5c3bc1d
--- /dev/null
+++ b/modules/caddyhttp/metrics_test.go
@@ -0,0 +1,66 @@
+package caddyhttp
+
+import (
+	"context"
+	"errors"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/prometheus/client_golang/prometheus/testutil"
+)
+
+func TestServerNameFromContext(t *testing.T) {
+	ctx := context.Background()
+	expected := "UNKNOWN"
+	if actual := serverNameFromContext(ctx); actual != expected {
+		t.Errorf("Not equal: expected %q, but got %q", expected, actual)
+	}
+
+	in := "foo"
+	ctx = contextWithServerName(ctx, in)
+	if actual := serverNameFromContext(ctx); actual != in {
+		t.Errorf("Not equal: expected %q, but got %q", in, actual)
+	}
+}
+
+func TestMetricsInstrumentedHandler(t *testing.T) {
+	handlerErr := errors.New("oh noes")
+	response := []byte("hello world!")
+	h := HandlerFunc(func(w http.ResponseWriter, r *http.Request) error {
+		if actual := testutil.ToFloat64(httpMetrics.requestInFlight); actual != 1.0 {
+			t.Errorf("Not same: expected %#v, but got %#v", 1.0, actual)
+		}
+		if handlerErr == nil {
+			w.Write(response)
+		}
+		return handlerErr
+	})
+
+	mh := middlewareHandlerFunc(func(w http.ResponseWriter, r *http.Request, h Handler) error {
+		return h.ServeHTTP(w, r)
+	})
+
+	ih := newMetricsInstrumentedHandler("foo", "bar", mh)
+
+	r := httptest.NewRequest("GET", "/", nil)
+	w := httptest.NewRecorder()
+
+	if actual := ih.ServeHTTP(w, r, h); actual != handlerErr {
+		t.Errorf("Not same: expected %#v, but got %#v", handlerErr, actual)
+	}
+	if actual := testutil.ToFloat64(httpMetrics.requestInFlight); actual != 0.0 {
+		t.Errorf("Not same: expected %#v, but got %#v", 0.0, actual)
+	}
+
+	handlerErr = nil
+	if err := ih.ServeHTTP(w, r, h); err != nil {
+		t.Errorf("Received unexpected error: %w", err)
+	}
+}
+
+type middlewareHandlerFunc func(http.ResponseWriter, *http.Request, Handler) error
+
+func (f middlewareHandlerFunc) ServeHTTP(w http.ResponseWriter, r *http.Request, h Handler) error {
+	return f(w, r, h)
+}
diff --git a/modules/caddyhttp/routes.go b/modules/caddyhttp/routes.go
index 10e0c9d..be23d39 100644
--- a/modules/caddyhttp/routes.go
+++ b/modules/caddyhttp/routes.go
@@ -157,7 +157,7 @@ func (routes RouteList) ProvisionHandlers(ctx caddy.Context) error {
 
 		// pre-compile the middleware handler chain
 		for _, midhandler := range routes[i].Handlers {
-			routes[i].middleware = append(routes[i].middleware, wrapMiddleware(midhandler))
+			routes[i].middleware = append(routes[i].middleware, wrapMiddleware(ctx, midhandler))
 		}
 	}
 	return nil
@@ -242,7 +242,13 @@ func wrapRoute(route Route) Middleware {
 // we need to pull this particular MiddlewareHandler
 // pointer into its own stack frame to preserve it so it
 // won't be overwritten in future loop iterations.
-func wrapMiddleware(mh MiddlewareHandler) Middleware {
+func wrapMiddleware(ctx caddy.Context, mh MiddlewareHandler) Middleware {
+	// first, wrap the middleware with metrics instrumentation
+	metricsHandler := newMetricsInstrumentedHandler(
+		serverNameFromContext(ctx.Context),
+		caddy.GetModuleName(mh),
+		mh,
+	)
 	return func(next Handler) Handler {
 		// copy the next handler (it's an interface, so it's
 		// just a very lightweight copy of a pointer); this
@@ -253,7 +259,7 @@ func wrapMiddleware(mh MiddlewareHandler) Middleware {
 		return HandlerFunc(func(w http.ResponseWriter, r *http.Request) error {
 			// TODO: This is where request tracing could be implemented
 			// TODO: see what the std lib gives us in terms of stack tracing too
-			return mh.ServeHTTP(w, r, nextCopy)
+			return metricsHandler.ServeHTTP(w, r, nextCopy)
 		})
 	}
 }
diff --git a/modules/metrics/adminmetrics.go b/modules/metrics/adminmetrics.go
new file mode 100644
index 0000000..f6a55e8
--- /dev/null
+++ b/modules/metrics/adminmetrics.go
@@ -0,0 +1,55 @@
+// Copyright 2020 Matthew Holt and The Caddy Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package metrics
+
+import (
+	"net/http"
+
+	"github.com/caddyserver/caddy/v2"
+)
+
+func init() {
+	caddy.RegisterModule(AdminMetrics{})
+}
+
+// AdminMetrics is a module that serves a metrics endpoint so that any gathered
+// metrics can be exposed for scraping. This module is not configurable, and
+// is permanently mounted to the admin API endpoint at "/metrics".
+// See the Metrics module for a configurable endpoint that is usable if the
+// Admin API is disabled.
+type AdminMetrics struct{}
+
+// CaddyModule returns the Caddy module information.
+func (AdminMetrics) CaddyModule() caddy.ModuleInfo {
+	return caddy.ModuleInfo{
+		ID:  "admin.api.metrics",
+		New: func() caddy.Module { return new(AdminMetrics) },
+	}
+}
+
+// Routes returns a route for the /metrics endpoint.
+func (m *AdminMetrics) Routes() []caddy.AdminRoute {
+	metricsHandler := createMetricsHandler(nil)
+	h := caddy.AdminHandlerFunc(func(w http.ResponseWriter, r *http.Request) error {
+		metricsHandler.ServeHTTP(w, r)
+		return nil
+	})
+	return []caddy.AdminRoute{{Pattern: "/metrics", Handler: h}}
+}
+
+// Interface guards
+var (
+	_ caddy.AdminRouter = (*AdminMetrics)(nil)
+)
diff --git a/modules/metrics/metrics.go b/modules/metrics/metrics.go
new file mode 100644
index 0000000..eab6342
--- /dev/null
+++ b/modules/metrics/metrics.go
@@ -0,0 +1,109 @@
+// Copyright 2020 Matthew Holt and The Caddy Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package metrics
+
+import (
+	"net/http"
+
+	"github.com/caddyserver/caddy/v2"
+	"github.com/caddyserver/caddy/v2/caddyconfig/caddyfile"
+	"github.com/caddyserver/caddy/v2/caddyconfig/httpcaddyfile"
+	"github.com/caddyserver/caddy/v2/modules/caddyhttp"
+
+	"go.uber.org/zap"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+)
+
+func init() {
+	caddy.RegisterModule(Metrics{})
+	httpcaddyfile.RegisterHandlerDirective("metrics", parseCaddyfile)
+}
+
+// Metrics is a module that serves a /metrics endpoint so that any gathered
+// metrics can be exposed for scraping. This module is configurable by end-users
+// unlike AdminMetrics.
+type Metrics struct {
+	metricsHandler http.Handler
+}
+
+// CaddyModule returns the Caddy module information.
+func (Metrics) CaddyModule() caddy.ModuleInfo {
+	return caddy.ModuleInfo{
+		ID:  "http.handlers.metrics",
+		New: func() caddy.Module { return new(Metrics) },
+	}
+}
+
+type zapLogger struct {
+	zl *zap.Logger
+}
+
+func (l *zapLogger) Println(v ...interface{}) {
+	l.zl.Sugar().Error(v...)
+}
+
+// Provision sets up m.
+func (m *Metrics) Provision(ctx caddy.Context) error {
+	log := ctx.Logger(m)
+	m.metricsHandler = createMetricsHandler(&zapLogger{log})
+	return nil
+}
+
+func parseCaddyfile(h httpcaddyfile.Helper) (caddyhttp.MiddlewareHandler, error) {
+	var m Metrics
+	err := m.UnmarshalCaddyfile(h.Dispenser)
+	return m, err
+}
+
+// UnmarshalCaddyfile sets up the handler from Caddyfile tokens. Syntax:
+//
+//     metrics <matcher>
+//
+func (m *Metrics) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
+	for d.Next() {
+		args := d.RemainingArgs()
+		if len(args) > 0 {
+			return d.ArgErr()
+		}
+	}
+	return nil
+}
+
+func (m Metrics) ServeHTTP(w http.ResponseWriter, r *http.Request, next caddyhttp.Handler) error {
+	m.metricsHandler.ServeHTTP(w, r)
+	return next.ServeHTTP(w, r)
+}
+
+// Interface guards
+var (
+	_ caddy.Provisioner           = (*Metrics)(nil)
+	_ caddyhttp.MiddlewareHandler = (*Metrics)(nil)
+	_ caddyfile.Unmarshaler       = (*Metrics)(nil)
+)
+
+func createMetricsHandler(logger promhttp.Logger) http.Handler {
+	return promhttp.InstrumentMetricHandler(prometheus.DefaultRegisterer,
+		promhttp.HandlerFor(prometheus.DefaultGatherer, promhttp.HandlerOpts{
+			// will only log errors if logger is non-nil
+			ErrorLog: logger,
+
+			// Allow OpenMetrics format to be negotiated - largely compatible,
+			// except quantile/le label values always have a decimal.
+			EnableOpenMetrics: true,
+		}),
+	)
+}
diff --git a/modules/standard/imports.go b/modules/standard/imports.go
index b8ea7a8..bc2d955 100644
--- a/modules/standard/imports.go
+++ b/modules/standard/imports.go
@@ -11,4 +11,5 @@ import (
 	_ "github.com/caddyserver/caddy/v2/modules/caddytls/standardstek"
 	_ "github.com/caddyserver/caddy/v2/modules/filestorage"
 	_ "github.com/caddyserver/caddy/v2/modules/logging"
+	_ "github.com/caddyserver/caddy/v2/modules/metrics"
 )