Such a wonderful definition:
A metric is simultaneously 1) a designed artifact, 2) a lens through which we observe phenomena, and 3) way we set and monitor goals.
This post is an excellent discussion of how to design a metric, but also importantly, how to think of the lifecycle of a metric. This was a new thought to me, but immediately resonated. Metrics don't last forever: they are born of a need, and eventually are retired once they have achieved their goal.