gather metrics on Gitlab::Git::Blob#truncated?

A customer ran into a display issue they found hard to understand, where an internal limit on how large a diff we display content for on the front end caused them to be unable to interact with the diff during code review. The method in question is Blob::readable_text? but the source of the false return was Gitlab:Git::Blob#truncate? whose return value is calculated from the size of the blob versus the data.. in any event, we should track this!

At a minimum:

• counter
  • false
  • true
• histogram of blob sizes?

We could also track which projects/files/language(?) these oversized blobs are originating from.

Related info

Working issue (Note that the original issue reported in that issue appears to not be the same as the issue the customer was experiencing) Docs on patch collapsing on diffs