Macro internal_tracing::checkpoint

macro_rules! checkpoint {
    ($name:ident; $checkpoint:ident) => { ... };
    ($name:ident; $checkpoint:ident, {$($metadata:tt)+}) => { ... };
    ($name:ident; $checkpoint:ident, $time:expr) => { ... };
    ($name:ident; $checkpoint:ident, $time:expr, {$($metadata:tt)+}) => { ... };
    (|$name:ident; $checkpoint:ident, $time:expr, $metadata:expr) => { ... };
}

Capture the trace/checkpoint.