pageserver: better observability for slow wait_lsn (#11176)

# Problem

We leave too few observability breadcrumbs in the case where wait_lsn is
exceptionally slow.

# Changes

- refactor: extract the monitoring logic out of `log_slow` into
`monitor_slow_future`
- add global + per-timeline counter for time spent waiting for wait_lsn
- It is updated while we're still waiting, similar to what we do for
page_service response flush.
- add per-timeline counterpair for started & finished wait_lsn count
- add slow-logging to leave breadcrumbs in logs, not just metrics

For the slow-logging, we need to consider not flooding the logs during a
broker or network outage/blip.
The solution is a "log-streak-level" concurrency limit per timeline.
At any given time, there is at most one slow wait_lsn that is logging
the "still running" and "completed" sequence of logs.
Other concurrent slow wait_lsn's don't log at all.
This leaves at least one breadcrumb in each timeline's logs if some
wait_lsn was exceptionally slow during a given period.
The full degree of slowness can then be determined by looking at the
per-timeline metric.

# Performance

Reran the `bench_log_slow` benchmark, no difference, so, existing call
sites are fine.

We do use a Semaphore, but only try_acquire it _after_ things have
already been determined to be slow. So, no baseline overhead
anticipated.

# Refs

-
https://github.com/neondatabase/cloud/issues/23486#issuecomment-2711587222

libs/utils/benches/benchmarks.rs

@@ -49,7 +49,13 @@ pub fn bench_log_slow(c: &mut Criterion) {
         // performance too. Use a simple noop future that yields once, to avoid any scheduler fast
         // paths for a ready future.
         if enabled {
-            b.iter(|| runtime.block_on(log_slow("ready", THRESHOLD, tokio::task::yield_now())));
+            b.iter(|| {
+                runtime.block_on(log_slow(
+                    "ready",
+                    THRESHOLD,
+                    std::pin::pin!(tokio::task::yield_now()),
+                ))
+            });
         } else {
             b.iter(|| runtime.block_on(tokio::task::yield_now()));
         }