Clean Sidekiq metrics from multiproc dir on start

What does this MR do?

Currently, we don't clean up Sidekiq metrics from the multiproc dir, only from puma and unicorn.
It should be fixed.
Discussion: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31668#note_203671999

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• [-] Documentation created/updated or follow-up review issue created
• [-] Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Related to #65278 (closed)