fix: prevent duplicate Prometheus metrics from Python expfmt callbacks (#5761)

Signed-off-by: Keiven Chang <keivenchang@users.noreply.github.com>

lib/bindings/python/rust/prometheus_metrics.rs

@@ -64,20 +64,13 @@ impl RuntimeMetrics {
             })
         });
 
-        // Register the callback at this hierarchy level
+        // Register the callback at this hierarchy level only.
+        // Do NOT register on parent hierarchies - combined scrapes automatically
+        // traverse child registries and include their callbacks.
         self.hierarchy
             .get_metrics_registry()
             .add_expfmt_callback(callback_arc.clone());
 
-        // Also register at all parent hierarchy levels so the callback is accessible
-        // when prometheus_expfmt() is called on any parent (e.g., DRT)
-        let parents = self.hierarchy.parent_hierarchies();
-        for parent in parents.iter() {
-            parent
-                .get_metrics_registry()
-                .add_expfmt_callback(callback_arc.clone());
-        }
-
         Ok(())
     }
 }

lib/runtime/src/metrics.rs

@@ -1251,6 +1251,37 @@ mod test_metricsregistry_prefixes {
         );
     }
 
+    #[tokio::test]
+    async fn test_expfmt_callback_only_registered_on_endpoint_is_included_once() {
+        // Sanity test: if an expfmt callback is registered only on the endpoint registry,
+        // scraping from the root (DRT) should still include it exactly once via the
+        // child-registry traversal.
+        let drt = create_test_drt_async().await;
+        let namespace = drt.namespace("ns_expfmt_ep_only").unwrap();
+        let component = namespace.component("comp_expfmt_ep_only").unwrap();
+        let endpoint = component.endpoint("ep_expfmt_ep_only");
+
+        let metric_line = "dynamo_component_active_decode_blocks{dp_rank=\"0\"} 0\n";
+        let callback: PrometheusExpositionFormatCallback =
+            Arc::new(move || Ok(metric_line.to_string()));
+
+        endpoint
+            .get_metrics_registry()
+            .add_expfmt_callback(callback);
+
+        let output = drt.metrics().prometheus_expfmt().unwrap();
+        let occurrences = output
+            .lines()
+            .filter(|line| line == &metric_line.trim_end_matches('\n'))
+            .count();
+
+        assert_eq!(
+            occurrences, 1,
+            "endpoint-registered exposition callback should appear once, got {} occurrences\n\n{}",
+            occurrences, output
+        );
+    }
+
     #[tokio::test]
     async fn test_recursive_namespace() {
         // Create a distributed runtime for testing