@Override
    public void after(ExtensionContext context) throws Exception {
        delegate.after();
    }
Shuts down all threads created by this {@link Extension} asynchronously.
Call {@code rule.get().shutdownGracefully().sync()} if you want to wait for complete termination.
junit5/src/main/java/com/linecorp/armeria/testing/junit5/common/AbstractEventLoopGroupExtension.java