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>

libs/metrics/src/lib.rs

@@ -19,6 +19,7 @@ use once_cell::sync::Lazy;
 use prometheus::core::{
     Atomic, AtomicU64, Collector, GenericCounter, GenericCounterVec, GenericGauge, GenericGaugeVec,
 };
+pub use prometheus::local::LocalHistogram;
 pub use prometheus::opts;
 pub use prometheus::register;
 pub use prometheus::Error;