example-display-extender-plugin-hook-views-pre-render-module.php

<?php
/**
 * @file
 * Contains views_head_metadata.module.
 */

/**
 * Implements hook_views_pre_render().
 */
function views_head_metadata_views_pre_render(\Drupal\views\ViewExecutable $view) {
  // Get the current display.
  $display = $view->getDisplay();

  // And get the list of extenders for this display.
  $extenders = $display->getExtenders();
  if (!isset($extenders['head_metadata'])) {
    // If the id of our plugin is not in the list something is wrong.
    return;
  }

  // Retrieve the settings of our plugins using our custom plugin method.
  $metadata_values = $extenders['head_metadata']->getMetadataValues();

  // Add the metadata tag for the title value.
  if (!empty($metadata_values['title'])) {
    // Generate a tag name unique for this display.
    $tag_name = 'views:' . $view->id() . ':' . $view->current_display . ':title';
    $content = $metadata_values['title'];
    $tag = [
      '#tag' => 'meta',
      '#attributes' => [
        'name' => $tag_name,
        'content' => $content
      ],
    ];
    $view->element['#attached']['html_head'][] = [$tag, $tag_name];
  }

  // Add the metadata tag for the description value.
  if (!empty($metadata_values['description'])) {
    // Generate a tag name unique for this display.
    $tag_name = 'views:' . $view->id() . ':' . $view->current_display . ':description';
    $content = $metadata_values['description'];
    $tag = [
      '#tag' => 'meta',
      '#attributes' => [
        'name' => $tag_name,
        'content' => $content
      ],
    ];
    $view->element['#attached']['html_head'][] = [$tag, $tag_name];
  }

}