Add custom metric importer

What does this MR do?

Addresses #230537 (closed) and #230539 (closed)

This adds a database importer/synchronizer for dashboard custom metrics. This will be the main entry point after a change has been committed to a dashboard.

1. Change is committed to a dashboard
2. Worker will start and call Gitlab::Metrics::Dashboard::Importer.update_prometheus_metrics(dashboard_path, project)

Nothing uses this code yet and some small changes will most likely be needed once this is actually hooked up but in the interest of keeping MRs small I've broken this out.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability 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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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