Add metrics widget extension

What does this MR do and why?

For #338282 (closed), also addresses #244337 and #11945 (closed)

This MR recreates the old generic metrics widget using the new merge request reports framework.

Screenshots or screen recordings

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How to set up and validate locally

1. git checkout 338282-metrics-mr-report-widget
2. echo "Feature.enable(:refactor_mr_widgets_extensions)" | rails c
3. set up metrics reports for a project (or clone this test project)
4. run a pipeline in the project for the default branch that generates a metrics report
5. run a pipeline for another branch that changes the metrics in the report
6. open a merge request for that branch
7. the metrics widget should appear in the merge request

note: the old widget will appear below the new widget, this is expected while we're implementing the new widgets

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.