Add feature flag for index circuit breakers

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/168566



Merged-by: Terri Chu <tchu@gitlab.com>
Approved-by: Terri Chu <tchu@gitlab.com>
Reviewed-by: Terri Chu <tchu@gitlab.com>
Co-authored-by: John Mason <9717668-johnmason@users.noreply.gitlab.com>
Co-authored-by: John Mason <jmason@gitlab.com>