Ran: bundle exec rubocop --only RSpec/EmptyLineAfterSubject -a

We don't use phase restarts, as we use `preload_app`:
https://github.com/puma/puma/blob/master/README.md#clustered-mode
`:puma_phase` values will always be zero.

In some cases (during worker start) it's possible that
Puma.stats returns an empty hash for worker's last status. In
that case we just skip sampling of the worker until these
stats are available.

This sampler gathers Puma-specific metrics which can be used by
Prometheus then.