page_service: batching observability & include throttled time in smgr metrics (#9870)

This PR 

- fixes smgr metrics https://github.com/neondatabase/neon/issues/9925 
- adds an additional startup log line logging the current batching
config
- adds a histogram of batch sizes global and per-tenant
- adds a metric exposing the current batching config

The issue described #9925 is that before this PR, request latency was
only observed *after* batching.
This means that smgr latency metrics (most importantly getpage latency)
don't account for
- `wait_lsn` time 
- time spent waiting for batch to fill up / the executor stage to pick
up the batch.

The fix is to use a per-request batching timer, like we did before the
initial batching PR.
We funnel those timers through the entire request lifecycle.

I noticed that even before the initial batching changes, we weren't
accounting for the time spent writing & flushing the response to the
wire.
This PR drive-by fixes that deficiency by dropping the timers at the
very end of processing the batch, i.e., after the `pgb.flush()` call.

I was **unable to maintain the behavior that we deduct
time-spent-in-throttle from various latency metrics.
The reason is that we're using a *single* counter in `RequestContext` to
track micros spent in throttle.
But there are *N* metrics timers in the batch, one per request.
As a consequence, the practice of consuming the counter in the drop
handler of each timer no longer works because all but the first timer
will encounter error `close() called on closed state`.
A failed attempt to maintain the current behavior can be found in
https://github.com/neondatabase/neon/pull/9951.

So, this PR remvoes the deduction behavior from all metrics.
I started a discussion on Slack about it the implications this has for
our internal SLO calculation:
https://neondb.slack.com/archives/C033RQ5SPDH/p1732910861704029

# Refs

- fixes https://github.com/neondatabase/neon/issues/9925
- sub-issue https://github.com/neondatabase/neon/issues/9377
- epic: https://github.com/neondatabase/neon/issues/9376

test_runner/fixtures/metrics.py

@@ -173,6 +173,7 @@ PAGESERVER_PER_TENANT_METRICS: tuple[str, ...] = (
     counter("pageserver_tenant_throttling_count_accounted_finish"),
     counter("pageserver_tenant_throttling_wait_usecs_sum"),
     counter("pageserver_tenant_throttling_count"),
+    *histogram("pageserver_page_service_batch_size"),
     *PAGESERVER_PER_TENANT_REMOTE_TIMELINE_CLIENT_METRICS,
     # "pageserver_directory_entries_count", -- only used if above a certain threshold
     # "pageserver_broken_tenants_count" -- used only for broken