Fix active streams metric

furumi-server/src/metrics.rs

@@ -100,6 +100,24 @@ impl RequestTimer {
     }
 }
 
+/// An RAII guard that increments the ACTIVE_STREAMS gauge when created
+/// and decrements it when dropped. This ensures streams are correctly counted
+/// even if they terminate abruptly.
+pub struct ActiveStreamGuard;
+
+impl ActiveStreamGuard {
+    pub fn new() -> Self {
+        ACTIVE_STREAMS.inc();
+        Self
+    }
+}
+
+impl Drop for ActiveStreamGuard {
+    fn drop(&mut self) {
+        ACTIVE_STREAMS.dec();
+    }
+}
+
 /// Render all registered metrics in Prometheus text format.
 pub fn render_metrics() -> String {
     let encoder = TextEncoder::new();