`flow.kickoff().token_usage` only returned the last @listen method's
`CrewOutput.token_usage`, so multi-crew flows under-reported by a
factor of N and bare `LLM.call(...)` invocations were ignored
entirely. SDK totals therefore disagreed with the CrewAI Enterprise UI
(Wharf), which aggregates every LLM span.
Add a new `flow.usage_metrics` property that wires an
`LLMCallCompletedEvent` listener for the duration of `kickoff_async`.
The listener scopes to the active flow via the `current_flow_id`
contextvar (the event bus copies the context at emit time, so the
value the handler sees is the one set when the LLM call fired) and
normalizes the provider-specific usage dict into a `UsageMetrics`.
This covers every LLM call inside the flow — crew-led, tool-led, and
bare `LLM.call(...)` — and matches the UI totals 1:1.
c4476366ff