chore: warn out request rejection (#6222)

Signed-off-by: PeaBrane <yanrpei@gmail.com>

lib/runtime/src/pipeline/network/egress/push_router.rs

@@ -314,6 +314,11 @@ where
                 // Check if we actually have any instances at all
                 let all_instances = self.client.instance_ids();
                 if !all_instances.is_empty() {
+                    tracing::warn!(
+                        instance_id,
+                        total_workers = all_instances.len(),
+                        "Rejecting request: all workers are busy"
+                    );
                     return Err(PipelineError::ServiceOverloaded(
                         "All workers are busy, please retry later".to_string(),
                     )