code: public static long getClientCustomerId(AdWordsSession session) {
      String accountIdStr = session.getClientCustomerId();
  
      // clientCustomerId might be absent from AdWordsSession, e.g., for
      // ReportDefinitionService.getReportFields() and CustomerService.getCustomers().
      // In this case, use ONE virtual account to handle rate limit of ALL unknown accounts combined.
      if (accountIdStr == null) {
        return VIRTUAL_CID;
      }
  
      try {
        return Long.parseLong(accountIdStr.replace("-", ""));
      } catch (NumberFormatException e) {
        throw new RateLimiterException("Encountered invalid CID: " + accountIdStr, e);
      }
    }
docs: Get client customer ID from the adwords session, and convert it to Long type.
  
  @param session the AdWords session
  @return the client customer ID in the AdWords session, or VIRTUAL_CID if absent