code: public static LinkedBindingBuilder<HttpClientObserver> bindNewObserver(final Binder binder, final Annotation annotation)
      {
          return Multibinder.newSetBinder(binder, HttpClientObserver.class, annotation).addBinding();
      }
docs: Register a HttpClientObserver which observes only requests from a HttpClient with the given Guice binding annotation.
  @return the binding builder you should register with