Fix cleanup of prometheus files on server start

What does this MR do?

It fixes deletion of prometheus files when server starts. on_master_start lifecycle callback is used to assure this. In cluster environment this event maps to before_fork callback.

But there is a difference between Puma and Unicorn:

• Puma calls before_fork once when booting up master process
• Unicorn runs before_fork whenever new worker is spawned

To unify this behavior we call before_fork only once - we use this callback for deleting Prometheus files and in config/initializers/macos.rb. So calling it once matches our needs and also requires minimum change in the code.

TODO

• create omnibus MR for updating Puma and Unicorn template in a separate MR
• update cloud native https://gitlab.com/gitlab-org/build/CNG/blob/master/gitlab-unicorn/unicorn.rb - tracked in https://gitlab.com/charts/gitlab/issues/1403

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/62862

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