Refactor common metrics importer

What does this MR do?

EE side of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30012

Issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/63044

This MR refactors our common_metrics importer into a new pattern. This is done for a couple of reasons:

1. It allows us to more easily extend classes/modules for EE.
2. It establishes a new pattern that will help prevent class/module collision if/when we create more importers beyond common_metrics.
3. It moves code to lib/gitlab so we don't need to explicitly require the importers.

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