[Misc] Extend vLLM Metrics logging API

[SolitaryThinker]

Currently, vLLM handles all of its metric logging in a single StatsLogger class, which has a hardcoded Prometheus client and text output. We would like to support our own Prometheus client and other arbitrary loggers.

Solution (Thanks to @Yard1)

1. Clean up StatLogger API by introducing a base class exposing a def log(self, stats: Stats) entrypoint.

2. Modify LLMEngine and AsyncLLMEngine to take in a list of StatLoggers, and expose an API to allow the user to specify them during initialization and/or runtime (add_logger/remove_logger)

3. Break up the current StatLogger into a LoggingStatLogger, which uses the logging.Logger and PrometheusStatLogger, which handles Prometheus.

4. Add RayPrometheusStatLogger, which uses ray.util.metrics. ray.util.metrics is a thin wrapper over the Prometheus client and has the same Gauge, Counter etc. APIs. Overrides _base_client_library with corresponding ray.util.metrics library.

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[DarkLight1337]

To speed up the CI queue for #5905, I've cancelled the distributed tests for the latest CI run in this PR since they won't pass anyway until #5905 has been merged. Please merge main into your branch after that happens so that the CI can pass once again.

[Yard1]

Thanks, just some small nits!