Developer Metrics List UI

What does this MR do and why?

First iteration of the new Metrics List UI.

• Add MetricsList component: fetches metrics and renders the MetricsTableList component
• Add MetricsTableList component: renders the given metrics as a table. Metric type is rendered as a Label.
• Add a temporary fetchMetrics mock

Note this is not hooked up yet to anything, as the changes required to scaffold the new feature are still in MRs:

• Scaffold Observability Metrics feature (frontend) (!134414 - merged)
• Scaffold Observability Metrics feature (backend) (!134393 - merged)

Everything is behind :observability_metrics feature flag

Closes gitlab-org/opstrace/opstrace#2435 (closed)

Screenshots or screen recordings

How to set up and validate locally

NA

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.