Datadog on staging (#16982)

* Include Datadog buildpack

* Add heroku_app tag to statsd

app.json

@@ -5,6 +5,7 @@
     "NPM_CONFIG_PRODUCTION": "true"
   },
   "buildpacks": [
+    { "url": "https://github.com/DataDog/heroku-buildpack-datadog.git#1.21" },
     { "url": "heroku/nodejs" }
   ],
   "formation": {

lib/statsd.js

@@ -9,6 +9,7 @@ module.exports = new StatsD({
   prefix: 'docs.',
   mock,
   globalTags: {
-    app: 'docs'
+    app: 'docs',
+    heroku_app: process.env.HEROKU_APP_NAME
   }
 })