title: Get all distinct keys in objects
body: <p>Is there a way to get the distinct keys of <code>infos</code> objects ? in other words, the list <code>['lastname', 'firstname', 'birthday', 'city']</code></p>
  
  <pre><code>/* QUERY */
  db.users.find({})
  
  /* 1 */
  {
      "_id" : "ee4068dbbe0005ed23249f38ce691665",
      "infos" : {
          "lastname" : "MATOUB",
          "firstname" : "Lounes"
      },
  }
  
  /* 2 */
  {
      "_id" : "0005ed232ee4068dbbe49f38ce6968db",
      "infos" : {
          "birthday": "19560312",
          "lastname" : "DOMRANE",
          "firstname" : "Malika"
      },
  }
  
  /* 3 */
  {
      "_id" : "68dbbe49f38e4068dbbe49f38ce69384",
      "infos" : {
          "lastname" : "AZEM",
          "firstname" : "Slimane",
          "city": "Tizi Ouzou"
      },
  }
  </code></pre>