Add Metrics Dashboard settings API to FF

What does this MR do and why?

This MR gates the metrics dashboard settings API options 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)

How to set up and validate locally

The frontend MR disabling the UI for the settings has already merged. If you really want to manually test the before/after, you can remove the feature flag checks in app/views/projects/settings/operations/show.html.haml to expose the relevant UI fields to make auth easier.

1. In a project, navigate to Settings > Monitor to monkey around with the inputs for metrics/grafana. With this MR, they should just be silently ignored with remove_monitor_metrics is enabled.

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.