title: Mongoose: Group by per month with the timestamp
body: <p>How can I group by per month and do the average of qty, in mongoose, having only the field timestamp?</p>
  
  <p><code>
  {
  "timestamp": 1234567890,
  "qty": 3
  },
  {
  "timestamp": 09876543322,
  "qty": 5
  }</code></p>
  
  <p>I want to know the average on qty for each month.</p>