title: Mongoose Aggregation to array of arrays
body: <p>I have a document structure like this:</p>
  <pre class="lang-js prettyprint-override"><code>{
    readings: [
      { t: 'temperature', r: 130 },
      { t: 'humidity', r: 100 }
    ],
    created_at: '2021-01-05T10:28:49.070Z'
  },
  {
    readings: [
      { t: 'temperature', r: 123 },
      { t: 'humidity', r: 456 }
    ],
    created_at: '2021-01-05T10:32:50.727Z'
  }
  </code></pre>
  <p>I need to aggregate the documents for a particular reading type.
  For example, the result of aggregation for reading type with <code>temperature</code> should be like following:</p>
  <pre class="lang-js prettyprint-override"><code>[
    [130, '2021-01-05T10:28:49.070Z'],
    [123, '2021-01-05T10:32:50.727Z']
  ]
  </code></pre>
  <p>How can I do this with an aggregation pipeline or something?</p>