Objectives
Beginner

Does a milestone count toward the objective's completion?

Yes. A milestone reads 0% until it is marked done and 100% after, and it is averaged in with the metrics.

A milestone counts as 0% until it is marked done, then 100%, and it is averaged with the metric key results. An objective with two milestones and a metric at 50% reads 17% with nothing done and 83% with both done.