Macro trace_record 

macro_rules! trace_record {
    ($target:expr, $event:expr) => { ... };
    ($target:expr, $event:expr, $a:expr) => { ... };
    ($target:expr, $event:expr, $a:expr, $b:expr) => { ... };
}

Record one observation, evaluating its arguments only when the target is being traced.

Prefer this to calling record directly: the macro keeps argument evaluation behind the filter check, so a call site can compute a value that would itself be too expensive for the measured path.