feat: add pool stats to periodic metrics log

Append pool=alive/total to the 60-second metrics summary and shutdown
log. Pool health is now visible without waiting for health test cycles.

src/s5p/server.py

@@ -192,7 +192,11 @@ async def _handle_client(
 # -- entry point -------------------------------------------------------------
 
 
-async def _metrics_logger(metrics: Metrics, stop: asyncio.Event) -> None:
+async def _metrics_logger(
+    metrics: Metrics,
+    stop: asyncio.Event,
+    pool: ProxyPool | None = None,
+) -> None:
     """Log metrics summary every 60 seconds."""
     while not stop.is_set():
         try:
@@ -200,7 +204,10 @@ async def _metrics_logger(metrics: Metrics, stop: asyncio.Event) -> None:
         except asyncio.TimeoutError:
             pass
         if not stop.is_set():
-            logger.info("metrics: %s", metrics.summary())
+            line = metrics.summary()
+            if pool:
+                line += f" pool={pool.alive_count}/{pool.count}"
+            logger.info("metrics: %s", line)
 
 
 async def serve(config: Config) -> None:
@@ -247,13 +254,17 @@ async def serve(config: Config) -> None:
         loop.add_signal_handler(sig, lambda s=sig: stop.set_result(s))
 
     metrics_stop = asyncio.Event()
-    metrics_task = asyncio.create_task(_metrics_logger(metrics, metrics_stop))
+    pool_ref = proxy_pool if isinstance(proxy_pool, ProxyPool) else None
+    metrics_task = asyncio.create_task(_metrics_logger(metrics, metrics_stop, pool_ref))
 
     async with srv:
         sig = await stop
         logger.info("received %s, shutting down", signal.Signals(sig).name)
         if isinstance(proxy_pool, ProxyPool):
             await proxy_pool.stop()
-        logger.info("metrics: %s", metrics.summary())
+        shutdown_line = metrics.summary()
+        if pool_ref:
+            shutdown_line += f" pool={pool_ref.alive_count}/{pool_ref.count}"
+        logger.info("metrics: %s", shutdown_line)
         metrics_stop.set()
         await metrics_task