Include websocket users metric (#14020)

This commit adds new metrics to know the number of registered users
we have in every WebSocket broadcasts hub

einterfaces/metrics.go

@@ -39,6 +39,8 @@ type MetricsInterface interface {
 	IncrementWebSocketBroadcast(eventType string)
 	IncrementWebSocketBroadcastBufferSize(hub string, amount float64)
 	DecrementWebSocketBroadcastBufferSize(hub string, amount float64)
+	IncrementWebSocketBroadcastUsersRegistered(hub string, amount float64)
+	DecrementWebSocketBroadcastUsersRegistered(hub string, amount float64)
 
 	AddMemCacheHitCounter(cacheName string, amount float64)
 	AddMemCacheMissCounter(cacheName string, amount float64)