Start tracking drift between main and viable/strict

[jathu]

To improve the reliability of CI, let's start measuring the drift between main and viable/strict. Once we have this, we should configure oncall alerts when it exceeds x days.

If you have context of how we can do this, please leave a comment. Specifically how we can get this into intern to configure oncall alerts.

cc @larryliu0820 @lucylq @mergennachin @byjlw