Automatic merge from submit-queue

Add apiserver metric for response sizes

Fixes https://github.com/kubernetes/kubernetes/issues/47728

This should help us understand GET/LIST call latencies better. It'll also help catch differences in object sizes across kubemark and real cluster.
I'm labelling the metrics by namespace (hoping that there won't be toooo many of them).

/cc @smarterclayton @gmarek 
cc @kubernetes/sig-scalability-misc @kubernetes/sig-api-machinery-misc @lavalamp @wojtek-t