monitoring: add very narrowly targeted sharing of observables

Created by: slimsag

Sharing and not sharing code can have substantial tradeoffs. My goal is for monitoring/ to be almost entirely declarative for simplicity/readability sake, because the real complexity is in what is being monitored.

That said, in rare circumstances sharing does make sense and so this PR introduces very narrowly targeted sharing of observable definitions and clearly documents when this should and shouldn't be done.