title: mongo convert field to NumberLong
body: <p>In mongo I want to convert for docs in a collection:</p>
  
  <pre><code>{   
   "timestamp" : 1494624438268.0
  }
  </code></pre>
  
  <p>to </p>
  
  <pre><code>{    
  "timestamp" : NumberLong(1494624438268)
  }
  </code></pre>
  
  <p>How can I do that? </p>