code: public <T> T attribute(Class<T> clazz) {
          return (T) config.inner.appAttributes.get(clazz);
      }
docs: Retrieve an attribute stored on the instance.
  Available on the {@link Context} through {@link Context#appAttribute}.
  Ex: app.attribute(MyExt.class).myMethod()
  Ex: ctx.appAttribute(MyExt.class).myMethod()