WIP #15098 measure stream

Pull request only here for me to have a place for question (impl is just a rough sketch, doubtful percision etc):

* Opinions about having something along the ways of timed being a "pattern"?
* Should this be a purely testing facility (moved into the streams TestKit?)? 
* Should I after merging to master also merge the MetricsKit into release-2.3 (I think yeah, but I'd rather ask anyway)?