Platform: Add Datadog monitoring & alerts [#77614326]

TODO: describe your changes

===

Pivotal tracker story [#77614326](https://www.pivotaltracker.com/story/show/77614326) in project *Platform*:

> https://github.com/HouseTrip/routemaster/issues/19
> Reuse the same service objects from the monitoring controller. 
Perhaps abstract this out in a spec helper that uses `around`?

It would be nice if it read

```
with_env do
  {
    'DATADOG_API_KEY' => 'api_key_super_secret', 
    'DATADOG_APP_KEY' => api_key_super_secret_app
  }
```