Track 'update_source_user_count' when all placeholders are kept as placeholders
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Work on this issue](https://contributors.gitlab.com/manage-issue?action=work&projectId=278964&issueIid=523494) - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=523494) </details> <!--IssueSummary end--> When keeping all placeholder users as placeholders, we should track updating the source user count in `app/services/import/source_users/keep_all_as_placeholder_service.rb` See the following discussion from !182679: - [ ] @SamWord started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/182679#note_2373438206): (+4 comments) > **Thought (non-blocking):** I wonder if there's any value in tracking `updated_source_user_count` here. It seems like it could be useful to see if it's a large number of users, as in customers are migrating lots of data but don't actually care about user reassignment for most imported users. WDYT? /
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