Automatic merge from submit-queue (batch tested with PRs 55792, 58342). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Add a metric to track usage of inflight request limit.

This one is tricky. The goal is to know how 'loaded' given apiserver is before we start dropping the load, to so we need to somehow expose 'fullness' of channels.

Sadly this metric is pretty volatile so it's not clear how to do this correctly. I decided to do pre-aggregation  to smoothen the metric a bit. In the current implementation the metric publishes maximum "usage" of the inflight is previous second.

If you have any ideas please share.
@smarterclayton @lavalamp @wojtek-t @liggitt @deads2k @caesarxuchao @sttts @crassirostris @hulkholden

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