monitoring: add monitoring package for Prometheus metric exports.

go.mod

@@ -14,6 +14,8 @@ require (
 	github.com/davecgh/go-spew v1.1.1
 	github.com/go-errors/errors v1.0.1
 	github.com/golang/protobuf v1.3.1
+	github.com/grpc-ecosystem/go-grpc-middleware v1.0.0
+	github.com/grpc-ecosystem/go-grpc-prometheus v1.2.0
 	github.com/grpc-ecosystem/grpc-gateway v0.0.0-20170724004829-f2862b476edc
 	github.com/jackpal/gateway v1.0.5
 	github.com/jackpal/go-nat-pmp v0.0.0-20170405195558-28a68d0c24ad
@@ -33,6 +35,7 @@ require (
 	github.com/lightningnetwork/lnd/ticker v1.0.0
 	github.com/ltcsuite/ltcd v0.0.0-20190101042124-f37f8bf35796
 	github.com/miekg/dns v0.0.0-20171125082028-79bfde677fa8
+	github.com/prometheus/client_golang v0.9.3
 	github.com/rogpeppe/fastuuid v0.0.0-20150106093220-6724a57986af // indirect
 	github.com/tv42/zbase32 v0.0.0-20160707012821-501572607d02
 	github.com/urfave/cli v1.18.0