Pipeline Health API - Traffic Light Status Heuristics

[roaksoax]

Part of #14463

Traffic-light Status Heuristics

We would like to get to a place where we can opinionatedly tell a user that either they don't have to worry about a pipeline (green), that they may need to dig in deeper soon (yellow), or that urgent intervention is required (red). Having historic metric flow-rates is going to be a big part of empowering opinionated heuristics. This effort is meant as a prerequisite for this future effort, and to provide the data with which to make such heuristics meaningful.

[robbavey]

Closed by #16520