Track requests_count metric

[dsalahutdinov]

Puma stats has the requests_count counter for each worker. https://github.com/puma/puma/blob/3e80f7c704e5585da4faa32edf0fa7a0abed3689/docs/stats.md#L96

It is very useful to determine is the problem of puma worker capacity degradation caused by increasing traffic. So yabeda-puma-plugin gem should provide it too.