Follow-up from "Include upload_duration in finalized fields"
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The following discussion from !71501 should be addressed:
- [ ] @iamricecake started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/71501#note_743880498):
> At the time when this was implemented, we thought there will always be only one file per request. Turns out we allow multiple files on some requests like for [designs](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/74377).
>
> We need to refactor this implementation to support multiple files in a single request.
Given we will sum up total duration per file, it would be great to also add `uploaded_file_count`.
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