Platform: Implement monitoring endpoints - GET topics [#74849038]

Pivotal tracker story [#74849038](https://www.pivotaltracker.com/story/show/74849038) in project *Platform*:

> https://github.com/HouseTrip/routemaster/issues/12
IMHO, whatever actually "does" what you test (here, `perform`) should always be repeated, even at the expense of DRYness: it's what you test, not something you do "before" you test to set up the stage.