Define reduce calculations needed to compute active editors per project family
Outcome of this ticket is a 1 pager design doc where the methodology we need to use to compute the metric is explained.
Here is the doc: https://docs.google.com/document/d/1JAO0TjzryBcgPZNYwsr7T7eyY7jPwJMj2FWp2_Dsbcg/edit