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

Author: Stanislav

Gopkg.lock

@@ -107,3 +107,7 @@ ignored = ["google.golang.org/appengine*"]
 [[override]]
   revision = "c10ba270aa0bf8b8c1c986e103859c67a9103061"
   name = "golang.org/x/oauth2"
+
+[[constraint]]
+  name = "github.com/prometheus/client_golang"
+  version = "0.9.0"

Gopkg.toml

@@ -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 =

custom/conf/app.ini.sample

@@ -302,6 +302,11 @@ Values containing `#` or `;` must be quoted using `` ` `` or `"""`.
 - `PULL`: **300**: Git pull from internal repositories timeout seconds.
 - `GC`: **60**: Git repository GC timeout seconds.
 
+## Metrics (`metrics`)
+
+- `ENABLED`: **false**: Enables /metrics endpoint for prometheus. 
+- `TOKEN`: **\<empty\>**: You need to specify the token, if you want to include in the authorization the metrics . The same token need to be used in prometheus parameters `bearer_token` or `bearer_token_file`.
+
 ## API (`api`)
 
 - `ENABLE_SWAGGER_ENDPOINT`: **true**: Enables /api/swagger, /api/v1/swagger etc. endpoints. True or false; default is true. 

docs/content/doc/advanced/config-cheat-sheet.en-us.md

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