title: How to retrieve meta from celery backend storage
body: <p>I am using celery with a flask app to launch some background tasks, I am using mongoDB as a backend.</p>
  
  <p>I would like to store in the backend some information about the task being launch and then be able to retrieve it.</p>
  
  <p>I believe the key is in the use of <code>self.update_state(state= ..., meta = {})</code> where meta is my custom information. However I do not find anything working.</p>