monitoring+cfg: add new option to export gRPC perf metrics

In this commit, we expose a new monitoring option to allow users to
export gRPC performance metrics. These metrics can be used to see how
long certain calls are taking, the total amount of time spent handling
calls, broken down by service and also call.

This option consumes additional memory and disk space for the Prometheus
server, which is why we're opting to make it an optional flag.

monitoring/monitoring_on.go

@@ -36,6 +36,14 @@ func ExportPrometheusMetrics(grpcServer *grpc.Server, cfg lncfg.Prometheus) erro
 
 		grpc_prometheus.Register(grpcServer)
 
+		// Enable the histograms which can allow plotting latency
+		// distributions of inbound calls. However we guard this behind
+		// another flag as this can generate a lot of additional data,
+		// as its a high cardinality metric typically.
+		if cfg.PerfHistograms {
+			grpc_prometheus.EnableHandlingTimeHistogram()
+		}
+
 		http.Handle("/metrics", promhttp.Handler())
 		go func() {
 			http.ListenAndServe(cfg.Listen, nil)