These spans require the user to pass the surrounding span, if any, when
entering a new span. They use the information inside (which is collector
specific) to track what asynchronous task is currently being executed.
Wrappers around `trace`, specific to an async library (e.g. Eio, lwt,
Async, etc.) can then smooth this over by providing a `with_async_span`
construct that uses some implicit contextual storage to carry the
`surrounding` scope around.
