Add metrics for internal API requests (#2310)

* Add response size and requests total to internal handler

* Move MustRegister calls to New* funcs

* Move MustRegister back to init

* Init at some place, minimize changes

syncapi/sync/requestpool.go

@@ -67,6 +67,9 @@ func NewRequestPool(
 	streams *streams.Streams, notifier *notifier.Notifier,
 	producer PresencePublisher,
 ) *RequestPool {
+	prometheus.MustRegister(
+		activeSyncRequests, waitingSyncRequests,
+	)
 	rp := &RequestPool{
 		db:       db,
 		cfg:      cfg,
@@ -183,12 +186,6 @@ func (rp *RequestPool) updateLastSeen(req *http.Request, device *userapi.Device)
 	rp.lastseen.Store(device.UserID+device.ID, time.Now())
 }
 
-func init() {
-	prometheus.MustRegister(
-		activeSyncRequests, waitingSyncRequests,
-	)
-}
-
 var activeSyncRequests = prometheus.NewGauge(
 	prometheus.GaugeOpts{
 		Namespace: "dendrite",