code: func (m *Meta) DDLJobQueueLen(jobListKeys ...JobListKeyType) (int64, error) {
  	listKey := m.jobListKey
  	if len(jobListKeys) != 0 {
  		listKey = jobListKeys[0]
  	}
  	return m.txn.LLen(listKey)
  }
docs: // DDLJobQueueLen returns the DDL job queue length.
  // The length of jobListKeys can only be 1 or 0.
  // If its length is 1, we need to replace m.jobListKey with jobListKeys[0].
  // Otherwise, we use m.jobListKey directly.