title: Dropping Events in a Custom serializer
body: <p>i have a Custom Serializer on an ElasticSearch sink.
  Based on some field values, which i read in the serializer, i might have the need to drop the event.</p>
  
  <p>I haven't found a way how to do that.
  Any ideas?</p>
  
  <p>thanks,</p>
  
  <p>Helmut</p>