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.

lncfg/monitoring_off.go

@@ -0,0 +1,19 @@
+// +build !monitoring
+
+package lncfg
+
+// Prometheus configures the Prometheus exporter when monitoring is enabled.
+// Monitoring is currently disabled.
+type Prometheus struct{}
+
+// DefaultPrometheus is the default configuration for the Prometheus metrics
+// exporter when monitoring is enabled. Monitoring is currently disabled.
+func DefaultPrometheus() Prometheus {
+	return Prometheus{}
+}
+
+// Enabled returns whether or not Prometheus monitoring is enabled. Monitoring
+// is currently disabled, so Enabled will always return false.
+func (p *Prometheus) Enabled() bool {
+	return false
+}