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. 
`age_of_oldest_message` is my method, `queue.message_count` is an attribute available from Bunny