Platform: Add Datadog monitoring & alerts [#77614326]

TODO: describe your changes

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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. 
Spaces around curly braces.

Also, shouldn't this test the output?
On way to do so is use dependency injection: the collector could take an optional `output: nil` parameter, defaulting to `$stdout`, and you could inject a `StringIO` in the test.