Function metrics 

pub fn metrics(
    prompt: u64,
    completion: u64,
    cached: u64,
    cost_usd: f64,
) -> Metrics

Convenience: build a metrics block with the three token counts the fidelity criterion (R17) requires on strict v1.7 agent steps with nonzero llm_call_count — in practice, every LLM-backed exported step.