Move cluster metrics to prometheus endpoint

What does this MR do?

This MR exposes a new cluster endpoint metrics_dashboard and sets up a prometheus proxy for clusters. This is done so cluster metrics can follow the same pattern we use for environment metrics, i.e. the front-end requests metrics_dashboard and then queries prometheus data via the prometheus proxy.

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

The old cluster metrics code-paths have not been changed and they should continue to function.

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