Show performance bar warnings

What does this MR do?

Following on from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31054, this presents the warnings from the backend to the user.

Right now, the performance bar is unopinionated: it will tell you how many SQL queries a page ran, but not if that's good or bad. It also hides away AJAX requests behind a dropdown, which means people are less likely to see those requests.

This adds a way of highlighting warnings generated by the backend in both places. In a request itself, a metric will have a warning item if the backend thinks it's too slow. And in the request selector, we highlight requests that have warnings associated.

Screenshots

This shows a request that has no warnings, but there are other requests that do have warnings:

When I open the request selector, I see the request with warnings:

When I pick that request, I see Gitaly warnings:

And hovering over those tells me why:

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/64179.