Macro impl_exported_metric_label_pair 

macro_rules! impl_exported_metric_label_pair {
    ($visibility:vis enum $label:ident { $($variant:ident => $value:tt),+ $(,)? }) => { ... };
    ($label:ty { $($variant:ident => ($index:expr, [($first_name:literal, $first_value:literal), ($second_name:literal, $second_value:literal)])),+ $(,)? }) => { ... };
}