Count native pool statement-cache misses and retired connections

Each connection of the native Postgres pool caches its prepared
statements. A miss costs a round trip to prepare, and the cache is
emptied whole when it reaches 512 statements. Nothing showed how often
either happens, or how often `txn_limit` replaces a connection and loses
its cache.

The pool now counts cache hits, prepares, cache clears, and connections
opened and retired. Python reports them per pool as
`synapse_rust_db_pool_statement_cache_hits_total`,
`..._statement_prepares_total`, `..._statement_cache_clears_total`,
`..._connections_opened_total` and `..._connections_retired_total`,
carried over when the native pool is closed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PVLsNh6akt4ifEizY4D1ta

scripts-dev/mypy_synapse_plugin.py

@@ -151,6 +151,7 @@ prometheus_metric_fullname_to_label_arg_map: Mapping[str, ArgLocation | None] =
     "synapse.metrics.jemalloc.JemallocCollector": None,
     "synapse.util.metrics.DynamicCollectorRegistry": None,
     "synapse.metrics.background_process_metrics._Collector": None,
+    "synapse.storage.rust_pool._RustPoolStatsCollector": None,
     #
     # `Metric` subclasses:
     "prometheus_client.metrics_core.Metric": None,