[Serve.llm] Add test to guard against upstream StatLogger changes

[eicherseiji]

Why are these changes needed?

RayPrometheusStatLogger inherits from StatLoggerBase in vLLM. We need to make sure this code path is exercised to ensure our implementation stays compliant with vLLM's abstract base class.

This is needed until vLLM 0.9.0, when we will switch to the implementation which landed in vllm-project/vllm#17925.

Related issue number

Guard against issue #53335, close LLM-1969

Checks

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• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[GeneDer]

[kouroshHakha]

We should proabably run this as a release test because the compute compatibility is limited on premerge. I don't think it is easy to change the hw of the premerge tests. but it's easy todo for release tests.