title: $push min of a document's own value and a constant value into its field
body: <p>Imagine a document like this:</p>
  
  <pre><code>{
    _id: 1,
    num: 2,
    history: []
  }
  </code></pre>
  
  <p>I'm $pushing an object into history and I want it to have its own <code>num: min(item.num + 1, 8)</code>. In this case we should end up with something like this:</p>
  
  <p><code>history: [ {event: 465, ... , num:3 } ]</code></p>