AS-389: Add per region totals to cloud metrics

This docstring is a huge improvement. Thanks!

Is there a particular place a caller of this thing can find documentation on it? That might be useful to link to as well. e.g. right now I know it contains ttl, but I don't know what key that's under, what format, what semantics... but OTOH it is also clearly not our job to document that, so if that is documented somewhere, a link seems appropriate.