Futures support for Zipkin tracing.
The Tracer type uses thread local storage to track the current span. This
works well in blocking applications where a unit of work "owns" a thread
while it's running. However, it is less appropriate for futures-based
applications where multiple distinct futures are being evaluated on the same
thread in an interleaved fashion.
This crate provides a wrapper Future which ensures that the thread-local
current span is set appropriately whenever the inner Future is running.