feat: connection rate and chain latency metrics

Add RateTracker (rolling deque, events/sec) and LatencyTracker (circular
buffer, p50/p95/p99 in ms) to the Metrics class.  Both are recorded in
_handle_client and exposed via summary(), to_dict(), /status, and /metrics.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/test_api.py

@@ -109,6 +109,8 @@ class TestHandleStatus:
         assert body["connections"] == 10
         assert body["success"] == 8
         assert "uptime" in body
+        assert "rate" in body
+        assert "latency" in body
 
     def test_with_pool(self):
         pool = MagicMock()
@@ -152,6 +154,8 @@ class TestHandleMetrics:
         assert body["connections"] == 42
         assert body["bytes_in"] == 1024
         assert "uptime" in body
+        assert "rate" in body
+        assert "latency" in body
 
 
 class TestHandlePool: