add array of meters and timers included

[jlambert121]

This is a workaround for #30 that is fully backward compatible. It allows you to split the metrics event and then assign the values to a known key using:

if "metric" in [tags] {
    split {
      field => "meters"
    }

    ruby {
      code => "event['meter'] = event[event['meters']]"
    }
  }

Thoughts?

This would potentially be superseded by #41.