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.

sample-lnd.conf

@@ -421,6 +421,12 @@
 ; Specify the interface to listen on for Prometheus connections.
 ; prometheus.listen=0.0.0.0:8989
 
+; If true, then we'll export additional information that allows users to plot
+; the processing latency, and total time spent across each RPC calls+service.
+; This generates additional memory load for the Prometheus server, and will end
+; up using more disk space over time.
+; prometheus.perfhistograms=true
+
 ; The alias your node will use, which can be up to 32 UTF-8 characters in
 ; length.
 ; alias=My Lightning ☇