code: public static <K, V> Marshaller<KeyValue<K, ? extends Iterable<V>>> getKeyValuesMarshaller(
        Marshaller<K> keyMarshaller, Marshaller<V> valueMarshaller) {
      return new KeyValuesMarshaller<>(keyMarshaller, valueMarshaller);
    }
docs: Returns a {@code Marshaller} for key-values pairs based on {@code keyMarshaller} and
  {@code valueMarshaller}.