code: func NewClientMetrics(stats registry) clientMetrics {
  	// Create the grpc prometheus client metrics instance and register it
  	grpcMetrics := grpc_prometheus.NewClientMetrics()
  	grpcMetrics.EnableClientHandlingTimeHistogram()
  	stats.MustRegister(grpcMetrics)
  
  	// Create a gauge to track in-flight RPCs and register it.
  	inFlightGauge := prometheus.NewGaugeVec(prometheus.GaugeOpts{
  		Name: "grpc_in_flight",
  		Help: "Number of in-flight (sent, not yet completed) RPCs",
  	}, []string{"method", "service"})
  	stats.MustRegister(inFlightGauge)
  
  	return clientMetrics{
  		grpcMetrics:  grpcMetrics,
  		inFlightRPCs: inFlightGauge,
  	}
  }
docs: // NewClientMetrics constructs a *grpc_prometheus.ClientMetrics, registered with
  // the given registry, with timing histogram enabled. It must be called a
  // maximum of once per registry, or there will be conflicting names.