Add panel for displaying diffs_batch.json metrics
We started tracking durations of certains parts of the diffs_batch.json endpoint (gitlab-org/gitlab#381432 (closed)) to see what we need to improve and monitor.
This updates the code review dashboard to include those metrics showing their average and P90.
Link to snapshot: https://dashboards.gitlab.net/dashboard/snapshot/L2SmlWVQxhF4eHzPkurA3HIO5wNI0GCP
Related to gitlab-org/gitlab#388522 (closed).
Edited by Patrick Bajao