title: Aggregation using $sample
body: <p>With an aggregation using <code>{ $sample: { size: 3 } }</code>, I'll get <code>3</code> random documents returned.</p>
  <p>How can I use a percentage of all documents instead?</p>
  <p>Something that'd look like <code>{ $sample: { size: 50% } }</code>?</p>