Prometheus endpoint (#5256)

* Add prometheus collector and route

* dep ensure -add github.com/prometheus/client_golang/prometheus

* dep ensure -update github.com/golang/protobuf

* add metrics to reserved usernames

* add comment head in metrics package

* fix style imports

* add metrics settings

* add bearer token check

* mapping metrics configs

* fix lint

* update config cheat sheet

* update conf sample, typo fix

models/user.go

@@ -698,6 +698,7 @@ var (
 		"issues",
 		"js",
 		"less",
+		"metrics",
 		"new",
 		"org",
 		"plugins",