code: public void getConfigurationInto(Entries configuration) {
      for (String prefix : prefixes) {
        configuration.setBoolean(prefix + ENABLE_SERVICE_ACCOUNTS_SUFFIX, isServiceAccountEnabled());
        if (getServiceAccountEmail() != null) {
          configuration.set(prefix + SERVICE_ACCOUNT_EMAIL_SUFFIX, getServiceAccountEmail());
        }
        if (getServiceAccountPrivateKeyId() != null) {
          configuration.set(
              prefix + SERVICE_ACCOUNT_PRIVATE_KEY_ID_SUFFIX, getServiceAccountPrivateKeyId());
        }
        if (getServiceAccountPrivateKey() != null) {
          configuration.set(
              prefix + SERVICE_ACCOUNT_PRIVATE_KEY_SUFFIX, getServiceAccountPrivateKey());
        }
        if (getServiceAccountKeyFile() != null) {
          configuration.set(prefix + SERVICE_ACCOUNT_KEYFILE_SUFFIX, getServiceAccountKeyFile());
        }
        if (getServiceAccountJsonKeyFile() != null) {
          configuration.set(prefix + JSON_KEYFILE_SUFFIX, getServiceAccountJsonKeyFile());
        }
        if (getClientId() != null) {
          configuration.set(prefix + CLIENT_ID_SUFFIX, getClientId());
        }
        if (getClientSecret() != null) {
          configuration.set(prefix + CLIENT_SECRET_SUFFIX, getClientSecret());
        }
        if (getOAuthCredentialFile() != null) {
          configuration.set(prefix + OAUTH_CLIENT_FILE_SUFFIX, getOAuthCredentialFile());
        }
        configuration.setBoolean(prefix + ENABLE_NULL_CREDENTIAL_SUFFIX, isNullCredentialEnabled());
      }
  
      // Transport configuration does not use prefixes
      if (getProxyAddress() != null) {
        configuration.set(PROXY_ADDRESS_KEY, getProxyAddress());
      }
      if (getProxyUsername() != null) {
        configuration.set(PROXY_USERNAME_KEY, getProxyUsername());
      }
      if (getProxyPassword() != null) {
        configuration.set(PROXY_PASSWORD_KEY, getProxyPassword());
      }
      configuration.set(HTTP_TRANSPORT_KEY, getTransportType().name());
    }
docs: Gets our parameters and fills it into the specified configuration.
  Typically the passed-in configuration is an empty
  instance of a configuration object that extends Entries.