[#431] metrics: Add missed label

Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

pkg/metrics/writecache.go

@@ -176,7 +176,7 @@ func newWCMethodDurationCounter(method string) *prometheus.HistogramVec {
 		Subsystem: wcSubsystem,
 		Name:      fmt.Sprintf("%s_req_duration_seconds", method),
 		Help:      fmt.Sprintf("Accumulated %s request process duration", method),
-	}, []string{wcShardID, wcSuccess})
+	}, []string{wcShardID, wcSuccess, wcStorage})
 }
 
 func newWCOperationCounterVec(operation string, labels []string) *prometheus.CounterVec {