code: func (mvcc *MVCCLevelDB) Get(key []byte, startTS uint64, isoLevel kvrpcpb.IsolationLevel) ([]byte, error) {
  	mvcc.mu.RLock()
  	defer mvcc.mu.RUnlock()
  
  	return mvcc.getValue(key, startTS, isoLevel)
  }
docs: // Get implements the MVCCStore interface.
  // key cannot be nil or []byte{}