feat(llm-observability): LangChain spans

[skoob13]

Problem

We want to capture the input and output of intermediary chains during agent workflows.

Changes

• Add the spans support for the LangChain callback handler.
• Replace the RunMetadata dict with dataclasses.
• Fix tests.

New fields:

• $ai_parent_id to capture a parent ID and build a tree.
• $ai_generation_id to capture a generation ID and unify API.
• $ai_span_id to capture an ID for a span.

[Twixes]

Suggested change

	log.warning(f"Run {run_id} is not a generation but attempted to be captured as a trace or span.")
	log.warning(f"Run {run_id} is a generation, but attempted to be captured as a trace or span.")

[Twixes]

Suggested change

	log.warning(f"Run {run_id} is not a generation but attempted to be captured as a generation.")
	log.warning(f"Run {run_id} is not a generation, but attempted to be captured as a generation.")

[Twixes]

This is one thing that gets me, should we be using $ai_span_id and $ai_span_name for both span and trace IDs/names after all? It feels pretty intuitive and definitely much simpler than somewhat duplicated properties (it'd be a breaking change, but it's a beta product)

[skoob13]

I thought about unifying them, too. Intuitively, I want to inherit $ai_generation and $ai_trace from $ai_span, so it makes sense to use $ai_span_id and $ai_span_name on all LLM events. We still want to keep $ai_trace_id and $ai_parent_id, though.

So I propose renaming $ai_trace_name (existing) to $ai_span_name, keeping the naming unified across all three events.

[skoob13]

Done

[Twixes]

LGTM

[Twixes]

Nit: A little redundant to suffix with Union

Suggested change

	RunMetadataUnion = Union[SpanMetadata, GenerationMetadata]
	RunMetadata = Union[SpanMetadata, GenerationMetadata]

[skoob13]

Oh yeah, I've missed that. There was a naming conflict before haha. I'll fix it!