Use separate Prometheus metrics dirs in dev/test

What does this MR do?

Store Sidekiq and Web server metrics from Prometheus in separate directories when in development or test environments.

How to check locally

Clean up the dir:
rm -rf tmp/prometheus_multiproc_dir/*

Start the server (e.g. gdk run), check that tmp/prometheus_multiproc_dir/sidekiq and tmp/prometheus_multiproc_dir/unicorn folders have been created and contain metric data.
Additionally, start with Puma: ./bin/web_puma start_foreground and check the similar for tmp/prometheus_multiproc_dir/puma folder.

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

Performance and testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Closes #63869 (closed)