Need to think about how that would get wired up (e.g. would yaml say `$brooklyn:sensor("brooklyn.entity.webapp.DynamicWebAppCluster", "webapp.reqs.perSec.windowed.perNode")` or is there a much nicer thing?

If we were referencing just statically defined sensors of the entity in question, that would be pretty simple and nice format. But if we're supporting sensors on other things (which makes sense if we can add enrichers) then it gets more complicated.

Could overload so `$brooklyn:sensor("webapp.reqs.perSec.windowed.perNode")` would look on the entity in question.

Anyway, it's late and I'm not sure what the answer is!