chore(llamaindex): test custom LLM instrumentation

[nirga]

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• [ ] I have added tests that cover my changes.
• [ ] If adding a new instrumentation or changing an existing one, I've added screenshots from some observability platform showing the change.
• [ ] PR name follows conventional commits format: feat(instrumentation): ... or fix(instrumentation): ....
• [ ] (If applicable) I have updated the documentation accordingly.