  MetadataMapVector* getSavedResponseMetadata() {
    if (saved_response_metadata_ == nullptr) {
      saved_response_metadata_ = std::make_unique<MetadataMapVector>();
    }
    return saved_response_metadata_.get();
  }
  MetadataMapVector* getSavedResponseMetadata() {
    if (saved_response_metadata_ == nullptr) {
      saved_response_metadata_ = std::make_unique<MetadataMapVector>();
    }
    return saved_response_metadata_.get();
  }