Function pack_row 

fn pack_row<'a>(
    packer: &'a mut PermutedRowPacker,
    metric_name: &str,
    metric_type: &str,
    labels: &[(String, String)],
    value: f64,
    help: &str,
    process_id: usize,
) -> (&'a RowRef, &'a RowRef)

Pack a metric row into key/value row pairs.