code: public function renderTableFooter()
      {
          $cells = [];
          foreach ($this->columns as $column) {
              /* @var $column Column */
              $cells[] = $column->renderFooterCell();
          }
          $content = Html::tag('tr', implode('', $cells), $this->footerRowOptions);
          if ($this->filterPosition === self::FILTER_POS_FOOTER) {
              $content .= $this->renderFilters();
          }
  
          return "<tfoot>\n" . $content . "\n</tfoot>";
      }
docs: Renders the table footer.
  @return string the rendering result.