title: How to aggregate sum in MongoDB to get a total count?
body: <p>For some collection with a field <code>{ wins: Number }</code>, how could I use <em>MongoDB Aggregation Framework</em> to get the total number of wins across all documents in a collection?</p>
  <h3>Example:</h3>
  <p>If I have 3 documents with <code>wins: 5</code>, <code>wins: 8</code>, <code>wins: 12</code> respectively, how could I use MongoDB Aggregation Framework to return the total number, i.e. <code>total: 25</code>.</p>