code: func (m *Meta) SetSchemaDiff(diff *model.SchemaDiff) error {
  	data, err := json.Marshal(diff)
  	if err != nil {
  		return errors.Trace(err)
  	}
  	diffKey := m.schemaDiffKey(diff.Version)
  	startTime := time.Now()
  	err = m.txn.Set(diffKey, data)
  	metrics.MetaHistogram.WithLabelValues(metrics.SetSchemaDiff, metrics.RetLabel(err)).Observe(time.Since(startTime).Seconds())
  	return errors.Trace(err)
  }
docs: // SetSchemaDiff sets the modification information on a given schema version.