Function extract_row 

fn extract_row<'a>(
    cols: &ColumnIndices,
    datums: &[Datum<'a>],
) -> (&'a str, Option<MetricKind>, bool, Vec<(&'a str, &'a str)>, Option<f64>, &'a str)

Extracts (metric_name, metric_kind, name_valid, sorted labels, value, help) from one shaped source row.

The planner already did the row-wise shaping.

Strings borrow from datums, so the caller must own what it needs (see SinkState::stage_ok) before the row backing datums is dropped.