Third-party-auth enhanced monitoring

[robrap]

We own third-party-auth (backend). [Another team owns AuthN MFE and third-party-auth frontend.]

This ticket is to add monitoring to detect highly exceptional changes (drops) in third-party-auth successful logins, or any idea that would have noticed that facebook login (specifically) was broken. The new runbook should note that we may not end up being the team to resolve the issue, depending on where the problem lies.