code: func (c *Cluster) Merge(regionID1, regionID2 uint64) {
  	c.Lock()
  	defer c.Unlock()
  
  	c.regions[regionID1].merge(c.regions[regionID2].Meta.GetEndKey())
  	delete(c.regions, regionID2)
  }
docs: // Merge merges 2 regions, their key ranges should be adjacent.