title: Filter message in event receiver WSO2
body: <p>I have recently started with WSO2 CEP. connected to a topic where many messages pass, but suppose I just want to get the messages of a type, for example all messages that have type TT -> {"name": " hello "," type ":" TT "}, I have seen that the messageSelector can be used but I do not know how to use it. An example->  type: 'TT'
  I do not know if I'm treating him well since the ones are json.</p>
  
  <p>Sorry for the English ;).</p>