Add Metrics Dashboard templates viewer to FF

What does this MR do and why?

This MR gates the metrics dashboard templates API and visibility behind the remove_monitor_metrics feature flag. Frontend is done, but we also want to deprecate/remove any API access.

Related issue: Metrics: Delete API entrypoints & helpers (#397138 - closed) (one of many MRs)

Context for removal:

• The Metrics Dashboard feature is being removed in %16.0.
  • Feature docs: https://docs.gitlab.com/ee/operations/metrics
  • Removal issue: Monitor: Metrics Deprecation and Removal (&10107)
• Most of the metrics dashboard frontend is currently behind the remove_monitor_metrics flag, but the flag has not yet been enabled. It needs to be enabled by default in time for %16.0.
  • The entire frontend should be behind the flag in order to enable it by default, but the API can merge after - as long as we make all breaking changes within %16.0.
  • Rollout issue: [Feature flag] Rollout of `remove_monitor_metrics` (#399248)
  • Tracking issue: Metrics: hide deprecated modules behind a featu... (#399231 - closed)
  • Enable-by-default MR: Set remove_monitor_metrics flag to true (!119989 - merged)

All related backend MRs to flag all metrics dashboard endpoints

• Add Metrics Dashboard annnotations API to FF (!120286 - merged)
• Add Metrics Dashboard starred dashboard API to FF (!120288 - merged)
• Add Metrics Dashboard templates viewer to FF (!120289 - merged)
• Add Metrics Dashboard environments API to FF (!120299 - merged)
• Add Metrics Dashboard settings API to FF (!120305 - merged)
• Add Metrics Dashboard GraphQL alert queries to FF (!120307 - merged)
• Add Metrics Dashboard embed pipeline to FF (!120310 - merged)
• Add Metrics Dashboard for internal dashboard en... (!120313 - merged)
• Add Metrics Dashboard base API to FF (!120316 - merged)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Open any project with a repository
2. Add a yml file to the ./gitlab/dashboards directory
3. See the lack of verification message above the file

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.