Track duration of specific parts of diffs request

As mentioned in create-stage#13067 (comment 1158100824), it may be helpful to track the duration of specific parts of diffs request.

For a start, I think we can get the duration of:

• serialization
• diffs collection (from DB or Gitaly)
• highlighting
• (open to suggestions)

I believe this can be done by utilizing prometheus metric (https://docs.gitlab.com/ee/development/prometheus_metrics.html).