Macro timing

macro_rules! timing {
    ($stat:expr, $ms:expr) => { ... };
    ($stat:path, $ms:expr) => { ... };
    ($stat:expr, $ms:expr; $( $key:literal => $value:literal ), *) => { ... };
    ($stat:path, $ms:expr; $( $key:literal => $value:literal ), *) => { ... };
    ($stat:expr, $ms:expr; $( $key:expr => $value:expr ), *) => { ... };
    ($stat:path, $ms:expr; $( $key:expr => $value:expr ), *) => { ... };
}

Send your own timing metric in milliseconds NOTE: Try to minimise variation in tag values (avoid things like timestamps or ids). See note in lib docs!