Async streams are not traced across threads

[dbolduc]

google-cloud-cpp/google/cloud/internal/async_read_write_stream_tracing.h

Lines 47 to 48 in d9ffe82

	auto start_span = internal::MakeSpan("Start");
	return impl_->Start().then(

We need to mark the stored span_ as the parent of these Start and Finish operations.

Otherwise, we get broken traces that show only Start and no Finish.

---

Tasks

Set parent for "Start" and "Finish" spans for...

• AsyncReadWriteRpcTracing
• AsyncStreamingReadRpcTracing
• AsyncStreamingWriteRpcTracing

[coryan]

Is this done then?

[dbolduc]

The functional stuff is done. There is a cleanup I want to do.

[dbolduc]

Nevermind about the cleanup. Closing.