[example] Add vLLM OpenAI-server example

[dimaioksha]

I was struggling when I was trying to add Langfuse support inside OpenAI-compatible vLLM server.
So here is an example what I found out and what worked for me and really helped for observability and tracking

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[dimaioksha]

Hello @maxdeichmann.
Can you make a review?
If you need any clarifications, feel free to ask me, I would like to help