Add extra_labelnames to PrometheusStatLogger for extensibility

[vraiti]

Purpose

Allow metrics to be extended with additional labels. This is useful when downstream use cases wish to collect vLLM metrics with enriched metadata

This came up in vLLM-Omni (vllm-project/vllm-omni#3362 (comment)): engines can be identified by two non-unique attributes: stage ID and replica ID. Adding extra_labelnames and extra_labelvalues to PrometheusStatLogger provides a clean mechanism for enriching vLLM metrics with this extra metadata.

Test Plan

Deploy vLLM-Omni with this patch of vLLM, using multiple replicas, and observe metric metadata correctly populating.

Test Result

Scraped from vLLM-Omni /metrics:
vllm_omni_metrics.txt

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[gemini-code-assist]

Code Review

This pull request introduces support for custom labels in the PrometheusStatLogger by adding extra_labelnames and extra_labelvalues parameters to its constructor. The implementation updates the metric initialization logic to dynamically include these additional labels across various engine metrics. Feedback was provided regarding a lack of validation for these new inputs; specifically, the code should ensure that both label names and values are provided together and that the number of values matches the number of names for each engine index to prevent ValueError or KeyError exceptions during metric registration.