Function export_timer 

pub fn export_timer<T: Tracer>(timer: &SubMsTimer, tracer: &T)
where
    T::Span: Span + Send + Sync + 'static,

Export a SubMsTimer timeline as a parent span + one child span per checkpoint. Span start times are reconstructed as now - elapsed_ns so the timing offsets the timer captured are preserved end-to-end.

The parent span’s name is the timer’s name (or "subms.timer" for an unnamed timer). Each checkpoint becomes a child span whose duration is the checkpoint’s since_last_ns.