[Serve] Add client IP address to HTTP access logs

[harshit-anyscale]

Summary

• Add client IP:port to Ray Serve HTTP access logs
• Thread client address from the proxy through the request context and metadata to the replica
• Handle both proxy-routed and direct ingress HTTP paths

For services behind a load balancer, uvicorn's ProxyHeadersMiddleware (enabled by default) resolves X-Forwarded-For into scope["client"] automatically, so the logged IP reflects the original client when FORWARDED_ALLOW_IPS is configured.

How It Works

The client IP is available at the entry point (proxy or direct ingress replica) but needs to reach the replica's access log, which runs in a separate process. The data flows through existing infrastructure:

External Client (10.0.91.46:54321)
       |
   [ Proxy ]
       |  1. Reads scope["client"] via proxy_request.client
       |  2. format_client_address() formats the raw tuple into "host:port"
       |  3. Logs it in the proxy access log
       |  4. Passes it into _RequestContext._client
       |
   [ DeploymentHandle ]
       |  5. default_impl.py copies _RequestContext._client → RequestMetadata._client
       |
   [ Replica ]
       |  6. Reads request_metadata._client and logs it in the replica access log

For direct ingress HTTP (replica serves HTTP directly, no proxy), the replica reads scope["client"] itself and formats it with the same format_client_address().

---

Update: Feature flag gating

Per review feedback, the client IP logging is now gated behind a feature flag that is off by default:

RAY_SERVE_LOG_CLIENT_ADDRESS=1

The gate is centralized in access_log_msg() in logging_utils.py — when the flag is off, the client parameter is ignored and the log format is unchanged from before this PR. The client address data still flows through the request context, but is simply not rendered in logs unless the flag is enabled.

Tests: Added a parametrized integration test (test_http_access_log_client_address) that verifies both flag states — client IP present when on, absent when off.

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[abrarsheikh]

let's gate this feature behind a feature flag, which is off by default.

[harshit-anyscale]

let's gate this feature behind a feature flag, which is off by default.

done.

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