title: Pandas groupby get value of a column where another column is minimum
body: <p>I have a dataframe <code>df</code> like this:</p>
  <div class="s-table-container">
  <table class="s-table">
  <thead>
  <tr>
  <th>region</th>
  <th>model</th>
  <th>metrics</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td>Tokyo</td>
  <td>ARIMA</td>
  <td>0.1</td>
  </tr>
  <tr>
  <td>Tokyo</td>
  <td>FeedForward</td>
  <td>0.2</td>
  </tr>
  <tr>
  <td>Tokyo</td>
  <td>DeepAR</td>
  <td>0.3</td>
  </tr>
  <tr>
  <td>Osaka</td>
  <td>ARIMA</td>
  <td>0.5</td>
  </tr>
  <tr>
  <td>Osaka</td>
  <td>FeedForward</td>
  <td>0.2</td>
  </tr>
  <tr>
  <td>Osaka</td>
  <td>DeepAR</td>
  <td>0.1</td>
  </tr>
  </tbody>
  </table>
  </div>
  <p>I want to group this by <code>region</code> and return the minimum value of <code>metrics</code> in each group, as well as the <code>model</code> value where the <code>metrics</code> is minimum.</p>
  <p>The expected result:</p>
  <pre><code>| region | model | metrics|
  | -------- | --------- |----|
  | Tokyo    | ARIMA |0.1|
  | Osaka    | DeepAR    |0.1|
  </code></pre>
  <p>I tried to do it like below, but not sure how I can complete:</p>
  <pre><code>df.groupby(&quot;region&quot;).agg({'metrics':'min', ####... })
  </code></pre>
  <p>Maybe use <code>argmin</code>? Any help will be appreciated. Thanks!</p>