code: func (i *Interceptor) HandleRPC(ctx context.Context, stat stats.RPCStats) {
  	lab, _ := ctx.Value(tagRPCKey).(prometheus.Labels)
  
  	switch in := stat.(type) {
  	case *stats.Begin:
  		if in.IsClient() {
  			i.monitoring.client.requests.With(lab).Inc()
  		} else {
  			i.monitoring.server.requests.With(lab).Inc()
  		}
  	case *stats.End:
  		if in.IsClient() {
  			i.monitoring.client.requests.With(lab).Dec()
  		} else {
  			i.monitoring.server.requests.With(lab).Dec()
  		}
  	}
  }
docs: // HandleRPC implements stats Handler interface.