This is required for launch. From an email:

The sign in team has built a standard, extensible way to track user interaction with our various signin promos across "access points" (i.e. places where users encounter signin). The metrics we record for each promo are:

User actions
Signin_Impression_From[AccessPoint] - recorded on promo impression
Signin_Signin_From[AccessPoint] - recorded on promo click

Histograms
Signin.SigninStartedAccessPoint.[Access point] - histogram recorded on promo click
Signin.SigninCompletedAccessPoint.[Access point] - histogram recorded on completion of signin flow