metrics: Enforce smaller set of method labels

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

2 files changed, 48 insertions, 2 deletions

diff --git a/modules/caddyhttp/metrics.go b/modules/caddyhttp/metrics.go
index 3e5d639..8aa9184 100644
--- a/modules/caddyhttp/metrics.go
+++ b/modules/caddyhttp/metrics.go
@@ -4,7 +4,6 @@ import (
 	"context"
 	"net/http"
 	"strconv"
-	"strings"
 	"sync"
 	"time"
 
@@ -109,7 +108,7 @@ func newMetricsInstrumentedHandler(handler string, mh MiddlewareHandler) *metric
 func (h *metricsInstrumentedHandler) ServeHTTP(w http.ResponseWriter, r *http.Request, next Handler) error {
 	server := serverNameFromContext(r.Context())
 	labels := prometheus.Labels{"server": server, "handler": h.handler}
-	method := strings.ToUpper(r.Method)
+	method := sanitizeMethod(r.Method)
 	// the "code" value is set later, but initialized here to eliminate the possibility
 	// of a panic
 	statusLabels := prometheus.Labels{"server": server, "handler": h.handler, "method": method, "code": ""}
@@ -160,6 +159,30 @@ func sanitizeCode(code int) string {
 	return strconv.Itoa(code)
 }
 
+// Only support the list of "regular" HTTP methods, see
+// https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods
+var methodMap = map[string]string{
+	"GET": http.MethodGet, "get": http.MethodGet,
+	"HEAD": http.MethodHead, "head": http.MethodHead,
+	"PUT": http.MethodPut, "put": http.MethodPut,
+	"POST": http.MethodPost, "post": http.MethodPost,
+	"DELETE": http.MethodDelete, "delete": http.MethodDelete,
+	"CONNECT": http.MethodConnect, "connect": http.MethodConnect,
+	"OPTIONS": http.MethodOptions, "options": http.MethodOptions,
+	"TRACE": http.MethodTrace, "trace": http.MethodTrace,
+	"PATCH": http.MethodPatch, "patch": http.MethodPatch,
+}
+
+// sanitizeMethod sanitizes the method for use as a metric label. This helps
+// prevent high cardinality on the method label. The name is always upper case.
+func sanitizeMethod(m string) string {
+	if m, ok := methodMap[m]; ok {
+		return m
+	}
+
+	return "other"
+}
+
 // taken from https://github.com/prometheus/client_golang/blob/6007b2b5cae01203111de55f753e76d8dac1f529/prometheus/promhttp/instrument_server.go#L298
 func computeApproximateRequestSize(r *http.Request) int {
 	s := 0
diff --git a/modules/caddyhttp/metrics_test.go b/modules/caddyhttp/metrics_test.go
index 6311935..78e380b 100644
--- a/modules/caddyhttp/metrics_test.go
+++ b/modules/caddyhttp/metrics_test.go
@@ -5,6 +5,7 @@ import (
 	"errors"
 	"net/http"
 	"net/http/httptest"
+	"strings"
 	"testing"
 
 	"github.com/prometheus/client_golang/prometheus/testutil"
@@ -82,3 +83,25 @@ type middlewareHandlerFunc func(http.ResponseWriter, *http.Request, Handler) err
 func (f middlewareHandlerFunc) ServeHTTP(w http.ResponseWriter, r *http.Request, h Handler) error {
 	return f(w, r, h)
 }
+
+func TestSanitizeMethod(t *testing.T) {
+	tests := []struct {
+		method   string
+		expected string
+	}{
+		{method: "get", expected: "GET"},
+		{method: "POST", expected: "POST"},
+		{method: "OPTIONS", expected: "OPTIONS"},
+		{method: "connect", expected: "CONNECT"},
+		{method: "trace", expected: "TRACE"},
+		{method: "UNKNOWN", expected: "other"},
+		{method: strings.Repeat("ohno", 9999), expected: "other"},
+	}
+
+	for _, d := range tests {
+		actual := sanitizeMethod(d.method)
+		if actual != d.expected {
+			t.Errorf("Not same: expected %#v, but got %#v", d.expected, actual)
+		}
+	}
+}