title: How to simply count the documents by a given criteria (mongo aggregation framework)?
body: <p>I would like to simply count the documents. What would be the correct way to do the following:</p>
  
  <pre><code>db.my_collection.aggregate({
      $match: { // go by the indexed field
          date: {
              $gte: new Date(2013,1,20),
              $lte: new Date(2013,1,27)
          }
      }
  },{
      $match: { // go by some other field
          someField: 'someValue'
      }
  },{
      $count: { // $sum? $group? $anythingElse?
          // ???????
      }
  })
  </code></pre>