code: public static <K, V> Marshaller<KeyValue<K, V>> getKeyValueMarshaller(Marshaller<K> keyMarshaller,
        Marshaller<V> valueMarshaller) {
      return new KeyValueMarshaller<>(keyMarshaller, valueMarshaller);
    }
docs: Returns a {@code Marshaller} for key-value pairs based on {@code keyMarshaller} and
  {@code valueMarshaller}.