Ports gitlab-ce#27608 to EE

What does this MR do?

• Updates the #metrics_dashboard endpoint (which is responsible for providing the FE with configuration for the metrics dashboard UI).
• Enables users to add yml templates to .gitlab/dashboards within their project repo, which will then be surfaced to the UI for rendering as a dashboard full of metrics (future MR for FE work).
• This work is behind 2 feature flags, as #metrics_dashboard is not yet in use (prometheus endpoint feature flag), and support for multiple dashboards will follow later (multiple dashboards feature flag).
• This MR is blocked by https://gitlab.com/gitlab-org/gitlab-ce/issues/60383, the work to setup the #metrics_dashboard endpoint

Issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/59974

Blocked by: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27405

CE MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27608

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