Sync create index for merge_request_metrics

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/142357



Merged-by: Gregory Havenga <11164960-ghavenga@users.noreply.gitlab.com>
Approved-by: Gregory Havenga <11164960-ghavenga@users.noreply.gitlab.com>
Co-authored-by: Martin Schurz <Martin.Schurz@telekom.de>