rpcserver+monitoring/config: enable Prometheus monitoring.

Start the Prometheus exporter in rpcserver.go if monitoring is enabled through the
build tag. Also allow users to specify what address they want the Prometheus
exporter to be listening on.

log.go

@@ -28,6 +28,7 @@ import (
 	"github.com/lightningnetwork/lnd/lnrpc/signrpc"
 	"github.com/lightningnetwork/lnd/lnrpc/walletrpc"
 	"github.com/lightningnetwork/lnd/lnwallet"
+	"github.com/lightningnetwork/lnd/monitoring"
 	"github.com/lightningnetwork/lnd/netann"
 	"github.com/lightningnetwork/lnd/routing"
 	"github.com/lightningnetwork/lnd/signal"
@@ -85,6 +86,7 @@ var (
 	irpcLog = build.NewSubLogger("IRPC", backendLog.Logger)
 	chnfLog = build.NewSubLogger("CHNF", backendLog.Logger)
 	chbuLog = build.NewSubLogger("CHBU", backendLog.Logger)
+	promLog = build.NewSubLogger("PROM", backendLog.Logger)
 )
 
 // Initialize package-global logger variables.
@@ -112,6 +114,7 @@ func init() {
 	invoicesrpc.UseLogger(irpcLog)
 	channelnotifier.UseLogger(chnfLog)
 	chanbackup.UseLogger(chbuLog)
+	monitoring.UseLogger(promLog)
 
 	addSubLogger(routerrpc.Subsystem, routerrpc.UseLogger)
 }
@@ -155,6 +158,7 @@ var subsystemLoggers = map[string]btclog.Logger{
 	"IRPC": irpcLog,
 	"CHNF": chnfLog,
 	"CHBU": chbuLog,
+	"PROM": promLog,
 }
 
 // initLogRotator initializes the logging rotator to write logs to logFile and