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.

lncfg/monitoring_on.go

@@ -13,6 +13,12 @@ type Prometheus struct {
 	// Enable indicates whether to export lnd gRPC performance metrics to
 	// Prometheus. Default is false.
 	Enable bool `long:"enable" description:"enable Prometheus exporting of lnd gRPC performance metrics."`
+
+	// PerfHistograms indicates if the additional histogram information for
+	// latency, and handling time of gRPC calls should be enabled. This
+	// generates additional data, and consume more memory for the
+	// Prometheus server.
+	PerfHistograms bool `long:"perfhistograms" description:"enable additional histogram to track gRPC call processing performance (latency, etc)"`
 }
 
 // DefaultPrometheus is the default configuration for the Prometheus metrics