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"
 )
 
@@ -80,13 +81,14 @@ func makeDownloadAPI(
 	activeRemoteRequests *types.ActiveRemoteRequests,
 	activeThumbnailGeneration *types.ActiveThumbnailGeneration,
 ) http.HandlerFunc {
-	handlerInfo := prometheus.NewCounterVec(
+	return promhttp.InstrumentHandlerCounter(
+		promauto.NewCounterVec(
 			prometheus.CounterOpts{
 				Name: name,
+				Help: "Total number of media_api requests for either thumbnails or full downloads",
 			},
-		[]string{name},
+			[]string{"code"},
-	)
+		), http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
-	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
@@ -107,5 +109,6 @@ func makeDownloadAPI(
 				activeThumbnailGeneration,
 				name == "thumbnail",
 			)
-	}))
+		},
+	))
 }