feat(metrics): add connection latency tracking

Author: Arqu

iroh/src/magicsock/endpoint_map/path_validity.rs

@@ -285,6 +285,9 @@ impl PathValidity {
 
         let quality = state.congestion_metrics.quality_score();
         metrics.path_quality_score.observe(quality);
+
+        let latency_ms = state.latency.as_secs_f64() * 1000.0;
+        metrics.connection_latency_ms.observe(latency_ms);
     }
 }
 
@@ -395,4 +398,36 @@ mod tests {
         let quality = validity.quality_score();
         assert!(quality < 0.9); // Should be penalized
     }
+
+    #[tokio::test]
+    async fn test_connection_latency_histogram() {
+        use crate::magicsock::Metrics as MagicsockMetrics;
+
+        let metrics = MagicsockMetrics::default();
+        let mut validity = PathValidity::new(Instant::now(), Duration::from_millis(10));
+
+        validity.record_metrics(&metrics);
+        assert_eq!(metrics.connection_latency_ms.count(), 1);
+
+        validity.update_pong(Instant::now(), Duration::from_millis(25));
+        validity.record_metrics(&metrics);
+        assert_eq!(metrics.connection_latency_ms.count(), 2);
+
+        validity.update_pong(Instant::now(), Duration::from_millis(50));
+        validity.record_metrics(&metrics);
+        assert_eq!(metrics.connection_latency_ms.count(), 3);
+
+        validity.update_pong(Instant::now(), Duration::from_millis(100));
+        validity.record_metrics(&metrics);
+        assert_eq!(metrics.connection_latency_ms.count(), 4);
+
+        let buckets = metrics.connection_latency_ms.buckets();
+        assert!(!buckets.is_empty());
+
+        let p50 = metrics.connection_latency_ms.percentile(0.5);
+        assert!(p50 > 10.0 && p50 < 100.0);
+
+        let p95 = metrics.connection_latency_ms.percentile(0.95);
+        assert!(p95 >= p50);
+    }
 }

iroh/src/magicsock/metrics.rs

@@ -77,6 +77,9 @@ pub struct Metrics {
     pub connection_handshake_success: Counter,
     /// Number of connections with a successful handshake that became direct.
     pub connection_became_direct: Counter,
+    /// Histogram of connection latency in milliseconds across all endpoint connections.
+    #[default(Histogram::new(vec![1.0, 5.0, 10.0, 25.0, 50.0, 100.0, 250.0, 500.0, 1000.0, f64::INFINITY]))]
+    pub connection_latency_ms: Histogram,
 
     /*
      * Path Congestion Metrics