Sam Brenner

[MLOB-1524] feat(llmobs): introduce LLM Observability SDK

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### What does this PR do? Introduces an LLM Observability SDK into the tracer, with its own tracing API extending the core tracer, and span tagging and processing functionality to...

[MLOB-1326] Add Node.js LLM Observability SDK docs

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### What does this PR do? What is the motivation? Adds Node.js SDK documentation for LLM Observability ### Merge instructions - [ ] Please merge after reviewing ### Additional notes

Streamed responses incompatible with multiple choices (`n>1`)

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### Checked other resources - [X] I added a very descriptive title to this issue. - [X] I searched the LangChain documentation with the integrated search. - [X] I used...