code: func (a *Annotation) MarshalJSON() ([]byte, error) {
  	return json.Marshal(&struct {
  		Timestamp int64  `json:"timestamp"`
  		Value     string `json:"value"`
  	}{
  		Timestamp: a.Timestamp.Round(time.Microsecond).UnixNano() / 1e3,
  		Value:     a.Value,
  	})
  }
docs: // MarshalJSON implements custom JSON encoding