Macro define_trace_id_fns 

macro_rules! define_trace_id_fns {
    ($subscriber:ty) => { ... };
}

Generates implementations of the functions pointed to by GET_TRACE_ID_FN and INSERT_TRACE_ID_FN given the type of the tracing::Subscriber. The caller (typically lexe_logger::try_init()) is responsible for initializing these statics using the generated implementations.

pub fn try_init() -> anyhow::Result<()> {
    FmtSubscriber::new().try_init().context("Logger already set")?;

    // Notice how `FmtSubscriber` here exactly matches our subscriber type.
    // If using a more complex subscriber, you will have to name the type,
    // e.g. `Layered<Filtered<FmtLayer<Registry, ...>, ..., ...>, ...>`.
    // See public/logger/src/lib.rs for an example of this.
    lexe_common::define_trace_id_fns!(FmtSubscriber);
    lexe_api::trace::GET_TRACE_ID_FN
        .set(get_trace_id_from_span)
        .map_err(|_| anyhow!("GET_TRACE_ID_FN already set"))?;
    lexe_api::trace::INSERT_TRACE_ID_FN
        .set(insert_trace_id_into_span)
        .map_err(|_| anyhow!("INSERT_TRACE_ID_FN already set"))?;

    Ok(())
}