title: Decompose Logstash json message into fields
body: <p>It have a logfile that stores event with a timestamp and a json message. For example:</p>
  
  <blockquote>
    <p>timestamp {"foo": 12, "bar": 13}</p>
  </blockquote>
  
  <p>I would like to decompose the keys (foo and bar) in the json part into fields in the Logstash output.</p>
  
  <p>I'm aware that I can set the format field in the Logstash file filter to json_event but in that case I have to include the timestamp in json. There is also a json filter, but that adds a single field with the complete json data structure, instead of using the keys.</p>
  
  <p>Any ideas how this can be done?</p>