Scaffold Observability Metrics feature (backend)

What does this MR do and why?

Scaffolding work required to introduces the new Observability Metrics feature.

• Add observability_metrics Feature Flag (rollout issue gitlab-org/opstrace/opstrace#2444 (closed))
• Create metrics_observability EE features
• Set up Rails routes, views, controllers
• Add project-level Navigation item Monitor > Metrics. Note there is UX Research ongoing about whether that's the best place for this (gitlab-org/opstrace&85 (comment 1582598980) , #416655 (closed)) Similarly to Tracing, we have permission to add the nav menu under Monitor behind FF while the research gets finalised.

Issue: metrics: Scaffold metrics UI (gitlab-org/opstrace/opstrace#2434 - closed)

FE: Scaffold Observability Metrics feature (frontend) (!134414 - merged)

Screenshots or screen recordings

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. In rails console enable the experiment fully

   Feature.enable(:observability_metrics)

2. Visit the project page such as https://local.gitlab.com:3443/flightjs/Flight/
3. Click on the Monitor > Metrics button

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.