private synchronized HttpResponse createRequest() {
            if (httpResponse != null) {
                throw new IllegalStateException("executed already");
            }
            executionStateUpdater.compareAndSet(this, ExecutionState.IDLE, ExecutionState.RUNNING);
            final HttpResponse newResponse = doCall(callFactory, request);
            httpResponse = newResponse;
            return newResponse;
        }
A {@link Factory} that creates a {@link Call} instance for {@link WebClient}.
retrofit2/src/main/java/com/linecorp/armeria/client/retrofit2/ArmeriaCallFactory.java