Macro map_metrics

macro_rules! map_metrics {
    ($namespace:ident
        @mem ($(($m_name:ident, $m_metric:ident, $m_desc:expr)),*)
    ) => { ... };
    (@define $namespace:ident
        @mem $(($m_name:ident, $m_metric:ident, $m_desc:expr)),*
    ) => { ... };
}