code: func (r *ReindexDestination) OpType(opType string) *ReindexDestination {
  	r.opType = opType
  	return r
  }
docs: // OpType specifies if this request should follow create-only or upsert
  // behavior. This follows the OpType of the standard document index API.
  // See https://www.elastic.co/guide/en/elasticsearch/reference/7.0/docs-index_.html#operation-type
  // for details.