docs: public IterableOfProtosSubject<?, M, Iterable<M>> valuesForKey(@NullableDecl Object key) {
      Subject.Factory<IterableValuesForKey<M>, Iterable<M>> factory =
          iterableOfProtos(
              "Values for key <" + key + "> (<" + actual() + ">) in " + actualAsString());
      return check().about(factory).that(((Multimap<Object, M>) actual()).get(key));
    }