Include format and status counters to individual controller actions

Currently, there are format and status counters summarized under `action_controller` i.e. `action_controller.format.html` & `action_controller.status.200`. I think it would be useful to have similar counters per controller action, so that it's easier to find individual actions that are behaving badly.

This patch adds status and format counters under each controller action i.e. `action_controller.controller.Admin.PostsController.index.status.200` & `action_controller.controller.Admin.PostsController.index.format.json`

I also:
- added a few more tests.
- locked minitest to 4.7 since 5.0 has some weird breaking changes.