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. 
I think this could be written as `topics.reduce(0) { |sum, topic| sum += topic.get_count }` or `topics.reduce { |sum, topic| sum += topic.get_count }` (the initial value can be omitted in this case).