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. 
Sorry I missed this before — doesn't this mean the scheduler will run on all dynos?

IMHO it should run on a separate worker dyno (you could provide a `bin/metric-reporter` executable to launch it), as otherwise it may interfere with event collection (and you'll get multiple reports to Datadog/logs).