Function metric_cases 

pub fn metric_cases(
    metric: &str,
    form: Form,
    runs: &[&EvalCases],
) -> Result<MetricCases, String>

One metric’s contributions over a run set’s case documents: cases[case][run]. Every document must list the same case ids in the same order (3.1.3); otherwise the set is refused (mixed-runs), never truncated or padded.