Trace metadata for observability tools (e.g. Langfuse, LangSmith).
Forwarded by OpenRouter to configured broadcast destinations. Common
keys include trace_id, trace_name, span_name, generation_name,
and parent_span_id; see the OpenRouter broadcast docs for the
current full set. Unknown keys are forwarded as custom metadata.
No environment-variable fallback — set per-call or on the constructor.
Example: {"trace_id": "abc-123", "span_name": "summarize"}
See https://openrouter.ai/docs/guides/features/broadcast/overview
