code: func NewServerMetrics(stats registry) serverMetrics {
  	// Create the grpc prometheus server metrics instance and register it
  	grpcMetrics := grpc_prometheus.NewServerMetrics()
  	grpcMetrics.EnableHandlingTimeHistogram()
  	stats.MustRegister(grpcMetrics)
  
  	// rpcLag is a prometheus histogram tracking the difference between the time
  	// the client sent an RPC and the time the server received it. Create and
  	// register it.
  	rpcLag := prometheus.NewHistogram(
  		prometheus.HistogramOpts{
  			Name: "grpc_lag",
  			Help: "Delta between client RPC send time and server RPC receipt time",
  		})
  	stats.MustRegister(rpcLag)
  
  	return serverMetrics{
  		grpcMetrics: grpcMetrics,
  		rpcLag:      rpcLag,
  	}
  }
docs: // NewServerMetrics registers metrics with a registry. It must be called a
  // maximum of once per registry, or there will be conflicting names.
  // It constructs and registers a *grpc_prometheus.ServerMetrics with timing
  // histogram enabled as well as a prometheus Histogram for RPC latency.