After moving the multiproc dir cleanup into `config.ru`:`warmup`, we
stopped cleaning Sidekiq metrics dir which is not correct.
This MR intended to fix that. More details:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31668
Use relative worker identifier for metrics (instead of Process.pid) and
identify when Unicorn/Puma/Sidekiq is used.
Previously, it was assumed that all metrics are gathered from Unicorn
due to hardcoded implementation which was incorrect.