AI-4795 Coverage for ray_vllm* metrics

[UTXOnly]

What does this PR do?

• Adds coverage for metrics prefixed with ray_vllm: for when vllm is run as a library
• Adds unit tests to ensure ray_vllm: metrics are collected

Motivation

• AI-4795

Review checklist (to be filled by reviewers)

• [] Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• [] Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[github-actions]

The changelog type changed or removed was used in this Pull Request, so the next release will bump major version. Please make sure this is a breaking change, or use the fixed or added type instead.

[github-actions]

The changelog type changed or removed was used in this Pull Request, so the next release will bump major version. Please make sure this is a breaking change, or use the fixed or added type instead.

[github-actions]

The changelog type changed or removed was used in this Pull Request, so the next release will bump major version. Please make sure this is a breaking change, or use the fixed or added type instead.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.93%. Comparing base (0ec4af3) to head (d2d5909).
Report is 2 commits behind head on master.

Additional details and impacted files

Flag	Coverage Δ
activemq	?
cassandra	?
hive	?
hivemq	?
ignite	?
jboss_wildfly	?
kafka	?
presto	?
solr	?
vllm	94.44% <100.00%> (+1.34%)	⬆️

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