Add support for DO_NOT_CLEAN_METRICS_DIR environment var

What does this MR do?

Support for the environment variable set in gitlab!21614 (closed) - prevent sidekiq and sidekiq-cluster from removing critical metrics file during initialization.

Related to gitlab#37387 (closed)

Related issues

Checklist

See Definition of done.

• Changelog entry created. Not applicable for Documentation changes and minor changes.
• Documentation created/updated
• Tests added
• [-] Integration tests added to GitLab QA, if applicable
• MR targeting master branch
• MR has a green pipeline on GitLab.com
• Equivalent MR/issue for CNG opened if applicable
• trigger-package has a green pipeline running against latest commit

Reviewer Checklist

In addition to above, reviewer must:

• Pipeline is green on dev.gitlab.org if the change is not touching documentation or internal cookbooks