Of particular note is that this change lets spans trace requests through `tower::Buffer` by internally carrying the `Span` at the time of `call` along with the request to the worker.
Creating a buffer can internally fail to spawn a worker. Before, that
error was returned immediately from `Buffer::new`. This changes `new` to
always return a `Buffer`, and the spawn error is encountered via
`poll_ready`.
Jon Gjengset
Sean McArthur
David Barsky
Carl Lerche