The metrics I find useful are things like trends before and after a change. For example, if lots of PRs are taking a long time to get through review because the descriptions don't get filled in well, I want to look at the time code spends in review before and after updating the PR template. Or before that, I want to see which teams have code in review for less time so I can look at their PR process and suggest changes to slower teams that the faster teams have already implemented. If a team is doing the same stuff as other teams but their typical PR size is much bigger I want to know if they have fewer stories that they should be breaking down further. And so on.
None of this is data I don't have a gut feel for, but having real numbers is useful for making a case for change. People don't always believe instinct. It's harder to argue with a well-designed graph.