Ensure diff collection limits are monitorable and configurable

Per me in https://gitlab.com/gitlab-org/create-stage/issues/41#note_203404643:

The number of diff files listed in a diff is limited based on the number of files, total number of lines, and total number of bytes of diffs rendered so far.

• Configurable: No, see https://gitlab.com/gitlab-org/gitlab-ce/issues/21852
• Documentation: Yes, see https://docs.gitlab.com/ee/development/diffs.html#diff-collection-limits

Per &1737, we would like this limit to be configurable and monitorable. I've asked for clarification on what that should look like in &1737 (comment 205413018).