Customizing traces and spans: Ability to add metadata to spans

[dhruvrnaik]

It would be great if we could customize span_data for any given span.

The metadata field only works for the top level trace, but child spans like response_span do not have any existing metadata (nor the ability to add such metadata) such as current_turn, agent_name etc that is useful for querying data.

I think a simple solution for now could be to use the AgentHooks to manipulate the current_span and add any metadata you like.

def on_llm_start(..., agent: str, ...):
    current_span = get_current_span() # response span
    current_span.span_data["custom_metadata"] = agent.name

To do this, all spans would need to start before the _start hooks are called and finish after the _end hooks are called.
This is not standardized currently. Agent span follows this pattern, while response span starts after the on_llm_start hook and ends before the on_llm_end hook, making it difficult to manipulate the span data.

[smf-h]

I dug into this and opened a fix in #2779.

This change now:

1. keeps the active
   esponse span current during both on_llm_start and on_llm_end,
2. lets hook code attach custom metadata that survives span export,
3. preserves compatibility with custom Model implementations that still use the older method signatures, and
4. finishes streamed
   esponse spans before tool/handoff processing so streamed traces match the non-streamed hierarchy.

I also added regression coverage for streamed and non-streamed hooks, exported metadata, legacy model signatures, and the streamed trace shape.