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
> 
> **TEST PLAN**
> https://routemaster-staging.herokuapp.com/topics (un: housetrip_avmczxf4dygleyjvudv6)
> and see cumulative counts for topics
I guess that `nil.to_i` already returns `0`, so `|| 0` is redundant.