feat: add quantiles() method to LatencyTracker for OpenMetrics

Returns {count, sum, 0.5, 0.95, 0.99} in seconds for Prometheus
summary exposition. Companion to the existing stats() (milliseconds).

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

src/s5p/metrics.py

@@ -67,6 +67,23 @@ class LatencyTracker:
             "p99": round(ms[min(int(n * 0.99), n - 1)], 1),
         }
 
+    def quantiles(self) -> dict | None:
+        """Return ``{count, sum, 0.5, 0.95, 0.99}`` in seconds, or None.
+
+        Designed for Prometheus/OpenMetrics summary exposition.
+        """
+        n = len(self._samples)
+        if n == 0:
+            return None
+        s = sorted(self._samples)
+        return {
+            "count": n,
+            "sum": sum(s),
+            "0.5": s[int(n * 0.50)],
+            "0.95": s[min(int(n * 0.95), n - 1)],
+            "0.99": s[min(int(n * 0.99), n - 1)],
+        }
+
 
 class Metrics:
     """Connection metrics.

tests/test_metrics.py

@@ -52,6 +52,23 @@ class TestLatencyTracker:
         assert s["min"] == 500.0
         assert s["max"] == 500.0
 
+    def test_quantiles_empty(self):
+        lt = LatencyTracker()
+        assert lt.quantiles() is None
+
+    def test_quantiles_seconds(self):
+        lt = LatencyTracker()
+        for i in range(1, 101):
+            lt.record(i / 1000)
+        q = lt.quantiles()
+        assert q is not None
+        assert q["count"] == 100
+        assert 0.050 <= q["0.5"] <= 0.052
+        assert 0.095 <= q["0.95"] <= 0.097
+        assert 0.099 <= q["0.99"] <= 0.101
+        assert "sum" in q
+        assert q["sum"] > 0
+
 
 # -- RateTracker -------------------------------------------------------------