Add metrics to track diff limiting

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Proposal

I've recently worked on a few issues (most immediately #225322 (closed) and #15829 (closed)) where we're changing the nature or application of diff size limits. We're doing this without having a solid picture of the current shape of the impact these limits have on users. We can get at some of this data through trawling through production DBs and making some guesses, but I think a more sustainable, useful way towards making metrics-driven decisions would be to begin tracking how often these limits are reached, creating a dashboard for seeing the trends over time.