Function with_usage_tracking 

pub fn with_usage_tracking(
    stream: LlmResponseStream,
    span: Span,
) -> LlmResponseStream

Wrap an LlmResponseStream so that the last UsageMetadata seen is recorded on the provided tracing span when the stream yields it.

The span is entered briefly for each item that carries usage metadata, ensuring adk_telemetry::record_llm_usage writes to the correct span regardless of which span is current when the stream is polled.

For non-streaming (single-response) calls the usage is recorded immediately. For streaming calls the usage typically arrives on the final chunk, so every chunk with usage_metadata overwrites the span fields (last write wins).