def summarize_spans(spans):
    return {'count': ..., 'error_count': ..., 'p95_ms': ..., 'total_tokens': ..., 'total_cost': ...}

1. count every span
2. keep failed spans in the snapshot
3. aggregate deterministic latency, token, and cost signals

• Use only the Python standard library.
• Return deterministic values so every run can be compared with the contract.
• Do not mutate caller-owned inputs.