[Doc] Update vllm example with metrics

[ronaldosaheki]

Complete the code from issue
#46360

Why are these changes needed?

Completes the example to expose vllm metrics following mixing the examples from issue #46360 and the test https://github.com/vllm-project/vllm/blob/7193774b1ff8603ad5bf4598e5efba0d9a39b436/tests/metrics/test_metrics.py#L342

The metrics prefix changes from vllm: to ray_vllm:

Dashboard https://docs.vllm.ai/en/latest/getting_started/examples/prometheus_grafana.html

Related issue number

Closes #46360

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.

[edoakes]

@akshay-anyscale PTAL!

[GeneDer]

Hi @ronaldosaheki thanks for contributing to this doc. We have recently released new llm serving APIs with Ray 2.43.0. You can check them out https://docs.ray.io/en/latest/serve/llm/overview.html We will soon be removing this doc to keep them more focused