Proper instrumenthandler fix

mediaapi/routing/routing.go

@@ -29,6 +29,7 @@ import (
 	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/matrix-org/util"
 	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 )
 
@@ -80,32 +81,34 @@ func makeDownloadAPI(
 	activeRemoteRequests *types.ActiveRemoteRequests,
 	activeThumbnailGeneration *types.ActiveThumbnailGeneration,
 ) http.HandlerFunc {
-	handlerInfo := prometheus.NewCounterVec(
+	return promhttp.InstrumentHandlerCounter(
-		prometheus.CounterOpts{
+		promauto.NewCounterVec(
-			Name: name,
+			prometheus.CounterOpts{
+				Name: name,
+				Help: "Total number of media_api requests for either thumbnails or full downloads",
+			},
+			[]string{"code"},
+		), http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
+			req = util.RequestWithLogging(req)
+
+			// Set common headers returned regardless of the outcome of the request
+			util.SetCORSHeaders(w)
+			// Content-Type will be overridden in case of returning file data, else we respond with JSON-formatted errors
+			w.Header().Set("Content-Type", "application/json")
+
+			vars := mux.Vars(req)
+			Download(
+				w,
+				req,
+				gomatrixserverlib.ServerName(vars["serverName"]),
+				types.MediaID(vars["mediaId"]),
+				cfg,
+				db,
+				client,
+				activeRemoteRequests,
+				activeThumbnailGeneration,
+				name == "thumbnail",
+			)
 		},
-		[]string{name},
+	))
-	)
-	return promhttp.InstrumentHandlerCounter(handlerInfo, http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
-		req = util.RequestWithLogging(req)
-
-		// Set common headers returned regardless of the outcome of the request
-		util.SetCORSHeaders(w)
-		// Content-Type will be overridden in case of returning file data, else we respond with JSON-formatted errors
-		w.Header().Set("Content-Type", "application/json")
-
-		vars := mux.Vars(req)
-		Download(
-			w,
-			req,
-			gomatrixserverlib.ServerName(vars["serverName"]),
-			types.MediaID(vars["mediaId"]),
-			cfg,
-			db,
-			client,
-			activeRemoteRequests,
-			activeThumbnailGeneration,
-			name == "thumbnail",
-		)
-	}))
 }