@Override
                        public void failed(final Exception ex) {
                            countDownLatch.countDown();
                            System.out.println(request + "->" + ex);
                        }
Example of asynchronous HTTP/1.1 request execution with message exchange multiplexing
over HTTP/2 connections.
httpclient5/src/test/java/org/apache/hc/client5/http/examples/AsyncClientH2Multiplexing.java