metrics: allow extra labels on HTTP request/response Prometheus metrics

[sshleifer]

Add an optional extra_labels: Optional[Dict[str, str]] parameter to add_prometheus_track_response_middleware that is merged into the label set of the sglang:http_requests_total / http_responses_total / http_requests_active metrics (and their inc/dec calls). This lets a deployment attach identifying labels (e.g. job id, replica id) to HTTP metrics so they can be filtered/joined the same way as the scheduler metrics, without a label-join. Defaults to no extra labels, so existing behavior is unchanged.

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Latest PR Test (Base): ❌ Run #29645532213
Latest PR Test (Extra): ❌ Run #29645532167

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[chatgpt-codex-connector]

Pass extra labels from the HTTP server

In the normal HTTP-server launch path, the only in-repo call remains add_prometheus_track_response_middleware(app) in python/sglang/srt/entrypoints/http_server.py:2370, so ServerArgs.extra_metric_labels never reaches this new parameter. Deployments using --extra-metric-labels will still get those labels on scheduler/tokenizer metrics but not on these HTTP counters/gauges, which defeats the new filtering/joining use case this parameter is meant to support.

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