message: fix span entry in hyper example
diff: @@ -79,7 +79,9 @@ fn echo(req: Request<Body>) -> Instrumented<BoxFut> {
  // future, waiting on concatenating the full body, so that
  // it can be reversed. Only then can we return a `Response`.
  (&Method::POST, "/echo/reversed") => {
  - let reversed = req.into_body().concat2().map(move |chunk| {
  + let span = span!("response", response_kind = "reversed");
  + let reversed = span.clone().enter(|| {
  + req.into_body().concat2().map(move |chunk| {
  let body = chunk.iter().rev().cloned().collect::<Vec<u8>>();
  event!(Level::Debug,
  {
  @@ -89,9 +91,10 @@ fn echo(req: Request<Body>) -> Instrumented<BoxFut> {
  "reversed request body");
  *response.body_mut() = Body::from(body);
  response
  + })
  });
  - (span!("response", response_kind = "reversed"), Box::new(reversed))
  + (span, Box::new(reversed))
  }
  // The 404 Not Found route...
  