title: Aggregate - $push an Array Expression in $group
body: <p>I have this aggregation pipeline:</p>
  
  <pre><code>{
      $project: {
          type: '$_id.type',
              datapoints: [
              {$multiply: ['$_id.timestamp', 1000]},
              '$count',
          ],
      },
  },
  {
      $group: {
          _id: {
              type: '$type',
          },
          datapoints: {$push: '$datapoints'},
      },
  },
  </code></pre>
  
  <p>... which brought me to a following question:<br>
  How could I possibly append/push/add/etc an array to <code>datapoints</code> in <code>$group</code> (what could let me avoid <code>$project</code> stage))?</p>
  
  <p>If I try putting the same expression for <code>datapoints</code> in the <code>$project</code> under <code>$push</code> in the <code>$group</code> it gives me an error</p>