title: MongoDB3.2. aggregation, remove processed document
body: <p>I have a collection <code>tickets</code> containing the docs with the scheme:</p>
  
  <pre><code>{_id: ObjectID, date: ISODate, cost: Number}
  </code></pre>
  
  <p>The following aggregation is done on the collection:</p>
  
  <pre><code>db.tickets.aggregate([
    {$match: {date: {$gte: ISODate("2016-02-01")}}},
    {$group:{ _id:"$date", totalCost: {$sum: "$cost"}, totalTickets: {$sum: 1}}}
  ], function(err,result) {
  // ...
  }
  </code></pre>
  
  <p>I want to randomly remove 20% of docs from the aggregation, means to simulate a situation that tickets collection contains 20% less docs than in reality.</p>
  
  <p>How can I do it?</p>