[Serve][3/N] Add application-level autoscaling snapshot

[nadongjun]

Description

Add application-level autoscaling snapshot support for observability.

This PR extends the existing deployment-level autoscaling snapshot feature (PR #56225) to support application-level autoscaling. When an app-level autoscaling policy is configured, the controller now emits ApplicationSnapshot logs containing aggregated metrics across all deployments in the application.

Related issues

Related to #55833

Additional information

bash % cat /tmp/ray/session_latest/logs/serve/autoscaling_snapshot_6668.log 
{"asctime": "2026-01-09 13:56:19,481", "levelname": "INFO", "message": "{'snapshots': [{'snapshot_type': 'application', 'timestamp_str': '2026-01-09T04:56:19Z', 'app': 'app_snap_1767934578', 'num_deployments': 2, 'total_current_replicas': 0, 'total_target_replicas': 2, 'scaling_status': 'scaling up', 'policy_name': 'ray.serve.tests.test_controller.simple_app_policy_for_test', 'errors': []}]}", "filename": "controller.py", "lineno": 511, "process": 6668, "timestamp_ns": 1767934579481838000}
{"asctime": "2026-01-09 13:56:19,999", "levelname": "INFO", "message": "{'snapshots': [{'snapshot_type': 'application', 'timestamp_str': '2026-01-09T04:56:19Z', 'app': 'app_snap_1767934578', 'num_deployments': 2, 'total_current_replicas': 2, 'total_target_replicas': 2, 'scaling_status': 'stable', 'policy_name': 'ray.serve.tests.test_controller.simple_app_policy_for_test', 'errors': []}]}", "filename": "controller.py", "lineno": 511, "process": 6668, "timestamp_ns": 1767934579999085000}

[gemini-code-assist]

Code Review

This pull request extends autoscaling observability to the application level by introducing ApplicationSnapshot logs. The changes are well-structured, reusing existing patterns from deployment-level snapshots, and include comprehensive tests. I've identified a couple of areas for improvement to enhance code clarity and correctness. Overall, this is a solid addition to Ray Serve's observability features.