code: func DefaultFormatter(fields logrus.Fields) logrus.Formatter {
  	for k, v := range logstashFields {
  		if _, ok := fields[k]; !ok {
  			fields[k] = v
  		}
  	}
  
  	return LogstashFormatter{
  		Formatter: &logrus.JSONFormatter{FieldMap: logstashFieldMap},
  		Fields:    fields,
  	}
  }
docs: // DefaultFormatter returns a default Logstash formatter:
  // A JSON format with "@version" set to "1" (unless set differently in `fields`,
  // "type" to "log" (unless set differently in `fields`),
  // "@timestamp" to the log time and "message" to the log message.
  //
  // Note: to set a different configuration use the `LogstashFormatter` structure.