[autoscaler][commands] Fix load metrics summary (#20623)

Running ray status with the changes from #20359
while running an autoscaler older than those changes
results in an error on input "head_ip" to LoadMetricsSummary.
See #20359 (comment)
This PR fixes the bug by restoring head_ip as an optional parameter of LoadMetricsSummary.

python/ray/autoscaler/_private/load_metrics.py

@@ -4,7 +4,7 @@ from functools import reduce
 import logging
 from numbers import Number
 import time
-from typing import Dict, List, Tuple
+from typing import Dict, List, Optional, Tuple
 
 import numpy as np
 import ray.ray_constants
@@ -34,6 +34,8 @@ class LoadMetricsSummary:
     # Counts of demand bundles requested by autoscaler.sdk.request_resources
     request_demand: List[DictCount]
     node_types: List[DictCount]
+    # Optionally included for backwards compatibility: IP of the head node.
+    head_ip: Optional[NodeIP] = None
 
 
 def add_resources(dict1: Dict[str, float],

python/ray/tests/test_resource_demand_scheduler.py

@@ -1409,6 +1409,12 @@ class LoadMetricsTest(unittest.TestCase):
         # Ensure summary_dict is json-serializable
         json.dumps(summary_dict)
 
+        # Backwards compatibility check: head_ip is correctly processed
+        # when included as an argument to LoadMetricsSummary.
+        summary_dict["head_ip"] = "1.1.1.1"
+        # No compatibility issue.
+        LoadMetricsSummary(**summary_dict)
+
 
 class AutoscalingTest(unittest.TestCase):
     def setUp(self):