Log diffs_files_count when MR diffs and commit are accessed
What does this MR do and why?
To be able to give an estimate on how much storage we are going to use when we cache diffs HTML, we need to get some data on how many files are there in diffs.
We add this logging metadata to MergeRequestsController#diffs
and
CommitController#show
actions since those are the actions that
we want to track.
MR acceptance checklist
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Related to #433999 (closed)