Depending on the value returned from QueueingHintFn, the scheduling queue changes how it queues each Pod:
  - if more than one QueueingHintFn returns QueueImmediately, it queues Pod to activeQ.
  - If no QueueingHintFn returns QueueImmediately and more than one plugin returns QueueAfterBackoff, it queues Pod to backoffQ if Pod is backing off, or to activeQ if Pod's backoff has already finished.

  - apiserver_flowcontrol_current_r: R(the time of the last change in state of the queues)
  - apiserver_flowcontrol_dispatch_r: R(the time of the latest request dispatch)
  - apiserver_flowcontrol_latest_s: S(the request last dispatched) = R(when that request starts executing in the virtual world)
  - apiserver_flowcontrol_next_s_bounds: min and max next S among non-empty queues

- The `priority_level_request_utilization` metric histogram is adjusted so that for the cases where `phase=waiting` the denominator is the cumulative capacity of all of the priority level's queues.

- Scheduler: added metrics to record the number of pending pods in different queues ([#75501](https://github.com/kubernetes/kubernetes/pull/75501), [@Huang-Wei](https://github.com/Huang-Wei))