code: func (t TagValue) Apply(err error) error {
  	if err == nil {
  		return nil
  	}
  	a := &Annotator{err, stackContext{frameInfo: stackFrameInfoForError(1, err)}}
  	return a.Tag(t).Err()
  }
docs: // Apply applies this tag value (key+value) directly to the error. This is
  // a shortcut for `errors.Annotate(err, "").Tag(t).Err()`.