- Some apiserver metrics were changed, as follows.
  - `priority_level_seat_count_samples` is replaced with `priority_level_seat_utilization`, which samples every nanosecond rather than every millisecond; the old metric conveyed utilization despite its name.
  - `priority_level_seat_count_watermarks` is removed.

- This PR adds the following metrics for API Priority and Fairness.
  - **apiserver_flowcontrol_priority_level_seat_count_samples**: histograms of seats occupied by executing requests (both regular and final-delay phases included), broken down by priority_level; the observations are taken once per millisecond.

    - `apiserver_flowcontrol_demand_seats_smoothed`: Smoothed seat demands
    - `apiserver_flowcontrol_target_seats`: Seat allocation targets
    - `apiserver_flowcontrol_seat_fair_frac`: Fair fraction of server's concurrency to allocate to each priority level that can use it

- Introduced new apiserver metric `apiserver_flowcontrol_current_inqueue_seats`. This metric is analogous to `apiserver_flowcontrol_current_inqueue_requests`, but tracks the total number of seats, as each request can take more than one seat. ([#119385](https://github.com/kubernetes/kubernetes/pull/119385), [@andrewsykim](https://github.com/andrewsykim))