title: Sum multiple instances of the same fields?
body: <p>Let's say I have data that represents geographic coordinates.</p>
  
  <pre><code>{"latitude" : 32.7874, "longitude" : -96.7989, "ip" : "1.1.1.1", "load" : 10}
  {"latitude" : 32.7874, "longitude" : -96.7989, "ip" : "2.2.2.2", "load" : 5}
  </code></pre>
  
  <p>Using db.collection.aggregate(), how would I achieve the result:</p>
  
  <pre><code>{"latitude" : 32.7874, "longitude" : -96.7989, "load" : 15}
  </code></pre>
  
  <p>Since both latitude and longitude have to be equal in order to sum, I am unsure how to use <code>$group</code></p>
  
  <p>I also know that <code>$project</code> will allow me to control which fields I get in the result, but I am unsure of how to use <code>$group</code> and <code>$sum</code> in conjunction with one another.</p>