Function on_begin_async_named_scope_dyn 

pub fn on_begin_async_named_scope_dyn(
    span_location: &'static SpanLocation,
    name: &str,
    parent_span_id: u64,
    depth: u32,
) -> u64

Like on_begin_async_named_scope, for a name built at runtime. The name is copied into the current block, so it needs to outlive only this call.