metrics: Always track method label in uppercase (#3742)

* metrics: Always track method label in uppercase

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

* Just use strings.ToUpper for clarity

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

metrics.go

@@ -1,6 +1,10 @@
 package caddy
 
 import (
+	"net/http"
+	"strconv"
+	"strings"
+
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/promauto"
 )
@@ -30,3 +34,44 @@ var adminMetrics = struct {
 	requestCount  *prometheus.CounterVec
 	requestErrors *prometheus.CounterVec
 }{}
+
+// Similar to promhttp.InstrumentHandlerCounter, but upper-cases method names
+// instead of lower-casing them.
+//
+// Unlike promhttp.InstrumentHandlerCounter, this assumes a "code" and "method"
+// label is present, and will panic otherwise.
+func instrumentHandlerCounter(counter *prometheus.CounterVec, next http.Handler) http.HandlerFunc {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		d := newDelegator(w)
+		next.ServeHTTP(d, r)
+		counter.With(prometheus.Labels{
+			"code":   sanitizeCode(d.status),
+			"method": strings.ToUpper(r.Method),
+		}).Inc()
+	})
+}
+
+func newDelegator(w http.ResponseWriter) *delegator {
+	return &delegator{
+		ResponseWriter: w,
+	}
+}
+
+type delegator struct {
+	http.ResponseWriter
+	status int
+}
+
+func (d *delegator) WriteHeader(code int) {
+	d.status = code
+	d.ResponseWriter.WriteHeader(code)
+}
+
+func sanitizeCode(s int) string {
+	switch s {
+	case 0, 200:
+		return "200"
+	default:
+		return strconv.Itoa(s)
+	}
+}