message: examples: fix echo example by reintroducing concurrency
diff: @@ -70,7 +70,7 @@ async fn main() -> Result<(), Box<dyn Error + Send + Sync + 'static>> {
  // Essentially here we're executing a new task to run concurrently,
  // which will allow all of our clients to be processed concurrently.
  - tokio::spawn(async move {
  + let fut = async move {
  let mut buf = [0; 1024];
  // In a loop, read data from the socket and write the data back.
  @@ -115,8 +115,8 @@ async fn main() -> Result<(), Box<dyn Error + Send + Sync + 'static>> {
  info!(message = "echo'd data", %peer_addr, size = n);
  }
  - })
  - .instrument(info_span!("echo", %peer_addr))
  - .await?;
  + }
  + .instrument(info_span!("echo", %peer_addr));
  + tokio::spawn(fut);
  }
  }
  