fix(rustproxy): prune stale per-route metrics, add per-route rate limiter caching and regex cache, and improve connection tracking cleanup to prevent memory growth

rust/crates/rustproxy-http/src/upstream_selector.rs

@@ -115,10 +115,18 @@ impl UpstreamSelector {
     /// Record that a connection to the given host has ended.
     pub fn connection_ended(&self, host: &str) {
         if let Some(counter) = self.active_connections.get(host) {
-            let prev = counter.value().fetch_sub(1, Ordering::Relaxed);
-            // Guard against underflow (shouldn't happen, but be safe)
+            let prev = counter.value().load(Ordering::Relaxed);
             if prev == 0 {
-                counter.value().store(0, Ordering::Relaxed);
+                // Already at zero — just clean up the entry
+                drop(counter);
+                self.active_connections.remove(host);
+                return;
+            }
+            counter.value().fetch_sub(1, Ordering::Relaxed);
+            // Clean up zero-count entries to prevent memory growth
+            if prev <= 1 {
+                drop(counter);
+                self.active_connections.remove(host);
             }
         }
     }
@@ -204,6 +212,31 @@ mod tests {
         assert_eq!(r4.host, "a");
     }
 
+    #[test]
+    fn test_connection_tracking_cleanup() {
+        let selector = UpstreamSelector::new();
+
+        selector.connection_started("backend:8080");
+        selector.connection_started("backend:8080");
+        assert_eq!(
+            selector.active_connections.get("backend:8080").unwrap().load(Ordering::Relaxed),
+            2
+        );
+
+        selector.connection_ended("backend:8080");
+        assert_eq!(
+            selector.active_connections.get("backend:8080").unwrap().load(Ordering::Relaxed),
+            1
+        );
+
+        // Last connection ends — entry should be removed entirely
+        selector.connection_ended("backend:8080");
+        assert!(selector.active_connections.get("backend:8080").is_none());
+
+        // Ending on a non-existent key should not panic
+        selector.connection_ended("nonexistent:9999");
+    }
+
     #[test]
     fn test_ip_hash_consistent() {
         let selector = UpstreamSelector::new();