Display actual count of merge requests in list
## Proposal Due to a query timeout issue in https://gitlab.com/gitlab-org/gitlab/-/work_items/563367 we capped the number in the tabs to `999`. This issue is to see what we can do to display a more accurate if not the number itself on the page since we display the count on the sidebar. The queries that do a count of all merge requests in the list [could reach over 100,000](https://gitlab.com/gitlab-org/gitlab/-/issues/563367#note_2926441571) so these queries can timeout resulting in an error. We could also consider loading the total count async and potentially placing that value in another area on the page https://gitlab.com/gitlab-org/gitlab/-/issues/563367#note_2927054551. There is an initiative to look at this pattern globally in lists with filters in https://gitlab.com/gitlab-org/gitlab-services/design.gitlab.com/-/work_items/3291+.
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