[Serve][Add] Include custom autoscaling policy name in logs (ray-project#57878)

## Description
Now, we hava custom autoscaling policy name in logs.

1. Deployment level
Test code from
https://docs.ray.io/en/master/serve/advanced-guides/advanced-autoscaling.html#custom-policy-for-deployment
<img width="1333" height="294" alt="image"
src="https://github.com/user-attachments/assets/bd26e576-fd3c-489b-94c2-4c11b25bb400"
/>

2. Application level
Test code from
https://docs.ray.io/en/master/serve/advanced-guides/advanced-autoscaling.html#application-level-autoscaling
<img width="1321" height="431" alt="image"
src="https://github.com/user-attachments/assets/d51f4952-faf5-47eb-80d0-6be357437505"
/>

## Related issues
Closes ray-project#57846

## Additional information

Signed-off-by: daiping8 <dai.ping88@zte.com.cn>
Signed-off-by: peterxcli <peterxcli@gmail.com>

Author: daiping8

python/ray/serve/_private/autoscaling_state.py

@@ -81,6 +81,13 @@ def register(self, info: DeploymentInfo, curr_target_num_replicas: int) -> int:
         self._target_capacity_direction = info.target_capacity_direction
         self._policy_state = {}
 
+        # Log when custom autoscaling policy is used for deployment
+        if not self._config.policy.is_default_policy_function():
+            logger.info(
+                f"Using custom autoscaling policy '{self._config.policy.policy_function}' "
+                f"for deployment '{self._deployment_id}'."
+            )
+
         return self.apply_bounds(target_num_replicas)
 
     def on_replica_stopped(self, replica_id: ReplicaID):
@@ -672,6 +679,13 @@ def register(
         self._policy = autoscaling_policy.get_policy()
         self._policy_state = {}
 
+        # Log when custom autoscaling policy is used for application
+        if not autoscaling_policy.is_default_policy_function():
+            logger.info(
+                f"Using custom autoscaling policy '{autoscaling_policy.policy_function}' "
+                f"for application '{self._app_name}'."
+            )
+
     def has_policy(self) -> bool:
         return self._policy is not None