pageserver: add tokio-epoll-uring slots waiters queue depth metrics (#9482)

In complement to
https://github.com/neondatabase/tokio-epoll-uring/pull/56.

## Problem

We want to make tokio-epoll-uring slots waiters queue depth observable
via Prometheus.

## Summary of changes

- Add `pageserver_tokio_epoll_uring_slots_submission_queue_depth`
metrics as a `Histogram`.
- Each thread-local tokio-epoll-uring system is given a `LocalHistogram`
to observe the metrics.
- Keep a list of `Arc<ThreadLocalMetrics>` used on-demand to flush data
to the shared histogram.
- Extend `Collector::collect` to report
`pageserver_tokio_epoll_uring_slots_submission_queue_depth`.

Signed-off-by: Yuchen Liang <yuchen@neon.tech>
Co-authored-by: Christian Schwarz <christian@neon.tech>

test_runner/fixtures/metrics.py

@@ -150,6 +150,7 @@ PAGESERVER_GLOBAL_METRICS: tuple[str, ...] = (
     counter("pageserver_tenant_throttling_count_accounted_finish_global"),
     counter("pageserver_tenant_throttling_wait_usecs_sum_global"),
     counter("pageserver_tenant_throttling_count_global"),
+    *histogram("pageserver_tokio_epoll_uring_slots_submission_queue_depth"),
 )
 
 PAGESERVER_PER_TENANT_METRICS: tuple[str, ...] = (