Scaffold Observability Logs feature (backend)

What does this MR do and why?

Scaffolding work required to introduces the new Observability Metrics feature.

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

Issue: Scaffold Logs UI (gitlab-org/opstrace/opstrace#2642 - closed)

Feature Mockup: gitlab-org/opstrace/opstrace#2611 (closed)

FE: !142990 (merged)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

How to set up and validate locally

1. Make sure you are logged in and have a Ultimate license

2. In rails console enable the experiment fully

   Feature.enable(:observability_logs)

3. Visit the project page such as https://local.gitlab.com:3443/flightjs/Flight/

4. Click on the Monitor > Logs button