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

custom/conf/app.ini.sample

@@ -659,3 +659,9 @@ FILE_EXTENSIONS = .adoc,.asciidoc
 RENDER_COMMAND = "asciidoc --out-file=- -"
 ; Don't pass the file on STDIN, pass the filename as argument instead.
 IS_INPUT_FILE = false
+
+[metrics]
+; Enables metrics endpoint. True or false; default is false.
+ENABLED = false
+; If you want to add authorization, specify a token here
+TOKEN =