Originaly from #62 (comment)

Alternatives:

• Document that it doesn't work. Adding a note doesn't hurt anyone, but I think the type annotations are pretty clear here and a type checker should catch this issue as well.
• Add a separate tracer.start_as_current_span_async() Support async/await syntax #62 (comment). IMO this is confusing because it would only be a decorator unlike start_as_current_span().