title: Does Firestore listener only transfer changed data?
body: <p>I would like to now what amount of data is send and billed with a Firestore document listener.</p>
  
  <ol>
  <li>Does the listener only sends changes to client or all the document ?</li>
  <li>What is the bill if I listen a 100Kb document and it changes of 1Kb ?</li>
  </ol>