code: func NewUIDOutput(field string, o Output) Output {
  	return OutputFunc(func(fields map[string]interface{}) error {
  		fields[field] = xid.New().String()
  		return o.Write(fields)
  	})
  }
docs: // NewUIDOutput returns an output filter adding a globally unique id (using github.com/rs/xid)
  // to all message going thru this output. The o parameter defines the next output to pass data
  // to.