Define observability_url in Gitlab::Observability

What does this MR do and why?

This MR moves the business logic from controller into Gitlab::Observability.

Note this feature is still behind a feature flag (see rollout issue).

Refs !103161 (comment 1162516727).

Screenshots

Still works ✅

http://localhost:3000/groups/gitlab-org/-/observability

How to set up and validate locally

Run specs.

• Run observability server in docker
• Configure your GDK
• Enable feature flag in your Rails console via Feature.enable(:observability_group_tab)
• Go to http://localhost:3000/groups/gitlab-org/-/observability

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.