Log message when upload via API exceeds limit

Since Workhorse started accelerating attachments in the upload API (!57250 (merged)), the content_length log in api_json.log no longer holds the actual bytes used by the uploaded file. It now reports the number of bytes from the Workhorse JSON metadata.

In order to determine which projects might be exceeding this limit, we log a JSON warning that will make it easier to fill in the allow list for https://gitlab.com/gitlab-org/gitlab/-/issues/325788.