Removed sync call in saving llm cost

[NickLucche]

Hey. thanks for your work maintaining this repo!
More like a question, but since the change is so small I thought it'd be faster to just make a PR.
Is there a reason why the llm cost indexing should be synchronous?
I didn't really need that feature so bad on my own use case, and it bothered me a little bit to have that await when serving responses.

Feel free to close this if the await is actually needed to have client synchronized :D

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