title: Meteor - Mongo aggregate does not have $count stage
body: <p>I am using <a href="https://github.com/meteorhacks/meteor-aggregate" rel="noreferrer">meteorhacks:aggregate</a> package to do Mongo aggregation in Meteor. I want to get the count at the last stage of the pipeline so I use this code:</p>
  
  <pre><code>Message.aggregate([
    {
      $match: {
        // ...
      }
    }, {
      $count: 'count'
    }
  ]);
  </code></pre>
  
  <p>It is pretty simple and should work, but I only get this error: </p>
  
  <pre><code>Exception while invoking method 'methodname' 
  MongoError: Unrecognized pipeline stage name: '$count'
  ...
  </code></pre>
  
  <p>Please help, thanks.</p>
  
  <p><strong>Updated</strong>: this question is not duplicated as an editor suggested, my main intention is to find out why I can not use <code>$count</code></p>