Ensure #[trace] macro works correctly for async functions.

[davidbarsky]

Original discussion.

@hawkw

Also for potential future work: it would be cool to allow things that aren't functions to be traced. At the very least, it would be nice to detect if we're annotating a future, and instrument it in the "right way" --- by constructing the span once and wrapping the future in a tokio_trace_futures::Instrumented future.
The naïve approach—just putting the #[trace] macro on the implementation of Future::poll for that future—wouldn't do the Right Thing; it would instead create a new span that's entered for every individual time that future is polled. This could potentially be a big enough gotcha that I think it's probably worthwhile to special-case this.
We'd probably want to figure out a similar approach for async fns as well...
[my comment redacted]
Yes, but I think we'd want to make sure we do the right thing for async fns --- namely, I think we'd want to make sure that the future created by that async fn is instrumented by that span, rather than just the initial call to the async fn that returns the future. This may require some testing.