metrics: Enforce smaller set of method labels

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

1 files changed, 25 insertions, 2 deletions

diff --git a/metrics.go b/metrics.go
index ab9d797..9a56f73 100644
--- a/metrics.go
+++ b/metrics.go
@@ -3,7 +3,6 @@ package caddy
 import (
 	"net/http"
 	"strconv"
-	"strings"
 
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/collectors"
@@ -47,7 +46,7 @@ func instrumentHandlerCounter(counter *prometheus.CounterVec, next http.Handler)
 		next.ServeHTTP(d, r)
 		counter.With(prometheus.Labels{
 			"code":   sanitizeCode(d.status),
-			"method": strings.ToUpper(r.Method),
+			"method": sanitizeMethod(r.Method),
 		}).Inc()
 	})
 }
@@ -76,3 +75,27 @@ func sanitizeCode(s int) string {
 		return strconv.Itoa(s)
 	}
 }
+
+// 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"
+}