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

Gopkg.lock

@@ -81,6 +81,14 @@
   pruneopts = "NUT"
   revision = "349dd0209470eabd9514242c688c403c0926d266"
 
+[[projects]]
+  branch = "master"
+  digest = "1:707ebe952a8b3d00b343c01536c79c73771d100f63ec6babeaed5c79e2b8a8dd"
+  name = "github.com/beorn7/perks"
+  packages = ["quantile"]
+  pruneopts = "NUT"
+  revision = "3a771d992973f24aa725d07868b467d1ddfceafb"
+
 [[projects]]
   digest = "1:67351095005f164e748a5a21899d1403b03878cb2d40a7b0f742376e6eeda974"
   name = "github.com/blevesearch/bleve"
@@ -405,11 +413,12 @@
   revision = "7f3990acf1833faa5ebd0e86f0a4c72a4b5eba3c"
 
 [[projects]]
-  digest = "1:b64f9be717fdab5f75122dc3868e8ca9d003779b6bc55f64f39a0cddc698bf88"
+  digest = "1:97df918963298c287643883209a2c3f642e6593379f97ab400c2a2e219ab647d"
   name = "github.com/golang/protobuf"
   packages = ["proto"]
   pruneopts = "NUT"
-  revision = "99511271042a09d1e01baea8781caa5210fec66e"
+  revision = "aa810b61a9c79d51363740d207bb46cf8e620ed5"
+  version = "v1.2.0"
 
 [[projects]]
   digest = "1:60e25fc5f5cfd7783f985ca99b4383e848981dddf0be584db7d809be20848e25"
@@ -573,6 +582,14 @@
   pruneopts = "NUT"
   revision = "c7c4067b79cc51e6dfdcef5c702e74b1e0fa7c75"
 
+[[projects]]
+  digest = "1:5985ef4caf91ece5d54817c11ea25f182697534f8ae6521eadcd628c142ac4b6"
+  name = "github.com/matttproud/golang_protobuf_extensions"
+  packages = ["pbutil"]
+  pruneopts = "NUT"
+  revision = "c12348ce28de40eed0136aa2b644d0ee0650e56c"
+  version = "v1.0.1"
+
 [[projects]]
   branch = "master"
   digest = "1:1ed0f3c066eb9d1c2ff7a864a6fa595c70b9b49049cc46af6a6f7ff0e4655321"
@@ -644,6 +661,51 @@
   pruneopts = "NUT"
   revision = "54653902c20e47f3417541d35435cb6d6162e28a"
 
+[[projects]]
+  digest = "1:aa2da1df3327c3a338bb42f978407c07de74cd0a5bef35e9411881dffd444214"
+  name = "github.com/prometheus/client_golang"
+  packages = [
+    "prometheus",
+    "prometheus/internal",
+    "prometheus/promhttp",
+  ]
+  pruneopts = "NUT"
+  revision = "1cafe34db7fdec6022e17e00e1c1ea501022f3e4"
+  version = "v0.9.0"
+
+[[projects]]
+  branch = "master"
+  digest = "1:2d5cd61daa5565187e1d96bae64dbbc6080dacf741448e9629c64fd93203b0d4"
+  name = "github.com/prometheus/client_model"
+  packages = ["go"]
+  pruneopts = "NUT"
+  revision = "5c3871d89910bfb32f5fcab2aa4b9ec68e65a99f"
+
+[[projects]]
+  branch = "master"
+  digest = "1:06375f3b602de9c99fa99b8484f0e949fd5273e6e9c6592b5a0dd4cd9085f3ea"
+  name = "github.com/prometheus/common"
+  packages = [
+    "expfmt",
+    "internal/bitbucket.org/ww/goautoneg",
+    "model",
+  ]
+  pruneopts = "NUT"
+  revision = "7e9e6cabbd393fc208072eedef99188d0ce788b6"
+
+[[projects]]
+  branch = "master"
+  digest = "1:102dea0c03a915acfc634b7c67f2662012b5483b56d9025e33f5188e112759b6"
+  name = "github.com/prometheus/procfs"
+  packages = [
+    ".",
+    "internal/util",
+    "nfs",
+    "xfs",
+  ]
+  pruneopts = "NUT"
+  revision = "185b4288413d2a0dd0806f78c90dde719829e5ae"
+
 [[projects]]
   branch = "master"
   digest = "1:5be01c22bc1040e2f6ce4755d51a0ac9cef823a9f2004fb1f9896a414ef519e6"
@@ -981,6 +1043,8 @@
     "github.com/nfnt/resize",
     "github.com/pquerna/otp",
     "github.com/pquerna/otp/totp",
+    "github.com/prometheus/client_golang/prometheus",
+    "github.com/prometheus/client_golang/prometheus/promhttp",
     "github.com/russross/blackfriday",
     "github.com/satori/go.uuid",
     "github.com/sergi/go-diff/diffmatchpatch",