apollo-portal/src/main/java/com/ctrip/framework/apollo/portal/component/RetryableRestTemplate.java/RetryableRestTemplate/assembleExtraHeaders
class RetryableRestTemplate:  private HttpHeaders assembleExtraHeaders(Env env) {
    String adminServiceAccessToken = getAdminServiceAccessToken(env);

    if (!Strings.isNullOrEmpty(adminServiceAccessToken)) {
      HttpHeaders headers = new HttpHeaders();
      headers.add(HttpHeaders.AUTHORIZATION, adminServiceAccessToken);
      return headers;
    }

    return null;
  }