v3rpc: add gRPC active streamsGauge

Documentation/metrics.md

@@ -90,6 +90,7 @@ All these metrics are prefixed with `etcd_grpc_`
 |--------------------------------|-------------------------------------------------------------------------------------|------------------------|
 | requests_total                 | Total number of received requests                                                   | Counter(method)        |
 | requests_failed_total                   | Total number of failed requests.                                                    | Counter(method,error)  |
+| active_streams                   | Total number of active streams.                                                    | Gauge(method)  |
 | unary_requests_duration_seconds     | Bucketed handling duration of the requests.                                         | Histogram(method)      |
 
 

etcdserver/api/v3rpc/interceptor.go

@@ -110,7 +110,9 @@ func metricsStreamInterceptor(srv interface{}, ss grpc.ServerStream, info *grpc.
 	service, method := splitMethodName(info.FullMethod)
 	receivedCounter.WithLabelValues(service, method).Inc()
 
+	streamsGauage.WithLabelValues(service, method).Inc()
 	err := handler(srv, ss)
+	streamsGauage.WithLabelValues(service, method).Dec()
 	if err != nil {
 		failedCounter.WithLabelValues(service, method, grpc.Code(err).String()).Inc()
 	}

etcdserver/api/v3rpc/metrics.go

@@ -33,6 +33,14 @@ var (
 			Help:      "Counter of failed requests.",
 		}, []string{"grpc_service", "grpc_method", "grpc_code"})
 
+	streamsGauage = prometheus.NewGaugeVec(
+		prometheus.GaugeOpts{
+			Namespace: "etcd",
+			Subsystem: "grpc",
+			Name:      "active_streams",
+			Help:      "Number of active streams.",
+		}, []string{"grpc_service", "grpc_method"})
+
 	handlingDuration = prometheus.NewHistogramVec(
 		prometheus.HistogramOpts{
 			Namespace: "etcd",
@@ -60,6 +68,7 @@ var (
 func init() {
 	prometheus.MustRegister(receivedCounter)
 	prometheus.MustRegister(failedCounter)
+	prometheus.MustRegister(streamsGauage)
 	prometheus.MustRegister(handlingDuration)
 
 	prometheus.MustRegister(sentBytes)