New Metric: Time waiting on reviewer(s)

[afranklin]

I especially love the "Open Time Trends" since I run a services team that's focused on response times for PRs from other teams. Because PRs are often assigned back to the submitter before closing, it would also be helpful to have a view like "Time waiting on reviewer" that would answer the question "For the team who owns the repo(s), how quickly are they responding to pull requests waiting on them?"

Thanks for this extension, Jeff, it's super helpful as is - this is just a wish. Cheers.