Track 'update_source_user_count' when all placeholders are kept as placeholders
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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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