title: Query and filter key names instead of values in MongoDB
body: <p>I want to find all key names from a collection that partially match a certain string.</p>
  
  <p>The closest I got was to check if a certain key exists, but that's an exact match:</p>
  
  <pre><code>db.collection.find({ "fkClientID": { $exists:1 }})
  </code></pre>
  
  <p>I'd like to get all keys that start with <code>fk</code> instead.</p>