docs: MultimapWithProtoValuesFluentAssertion<M> usingConfig(FluentEqualityConfig newConfig) {
      Subject.Factory<MultimapWithMessageValuesSubject<K, M>, Multimap<K, M>> factory =
          multimapWithMessageValues(newConfig);
      MultimapWithMessageValuesSubject<K, M> newSubject = check().about(factory).that(actual());
      if (internalCustomName() != null) {
        newSubject = newSubject.named(internalCustomName());
      }
      return new MultimapWithProtoValuesFluentAssertionImpl<>(newSubject);
    }