code: func (s *tikvStore) BeginWithStartTS(startTS uint64) (kv.Transaction, error) {
  	txn, err := newTikvTxnWithStartTS(s, startTS)
  	if err != nil {
  		return nil, errors.Trace(err)
  	}
  	metrics.TiKVTxnCounter.Inc()
  	return txn, nil
  }
docs: // BeginWithStartTS begins a transaction with startTS.