Reinitialize metrics files on webserver master start

What does this MR do?

Reinitializes metrics files on webserver master start,
to prevent the situation when the files is wiped by CleanupMultiprocDirService in warmup, while is still linked to the Sampler.

Related MR for Prometheus::Client: https://gitlab.com/gitlab-org/prometheus-client-mmap/merge_requests/40

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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