Add warnings to performance bar response

What does this MR do?

This adds warnings to the performance bar response data from the backend. It does not display them on the frontend (that's been extracted to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/32344).

The keys added are:

1. has_warnings - this a boolean is at the top level (alongside context and data).
2. data.$metric.warnings - an array of warnings (empty if no warnings) for the given metric (ActiveRecord, Gitaly, Redis).
3. data.$metric.details.$[].warnings - an array of warnings for a specific call (empty if no warnings). Currently this array can only have length zero or one, but we might add N+1 detection and similar in future.

Does this MR meet the acceptance criteria?

Performance and testing

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

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