Scaffold Metrics Details UI (backend)

What does this MR do and why?

Setup route (metrics/:id) and basic view for the Metrics Details UI.

Related FE changes: !137033 (merged)

Part of gitlab-org/opstrace/opstrace#2539 (closed)

(Note this is an Experimental feature. No users yet)

Screenshots or screen recordings

(Not much to see)

How to set up and validate locally

• Enable observability_metrics feature flag

Apply patch to load mocks ( pbpaste | git apply --allow-empty )

• Go to https://local.gitlab.com:3443/flightjs/Flight/-/metrics/foo.bar
• (optionally) Check the div is setup correctly from the Js console ( $('#js-observability-metrics-details')[0])

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.