metatag_custom_route.module

<?php

/**
 * @file
 * Contains metatag_custom_route.module.
 */

use Drupal\Core\Routing\RouteMatchInterface;
use Drupal\metatag_custom_route\Entity\MetatagRouteInterface;

/**
 * Implements hook_help().
 */
function metatag_custom_route_help($route_name, RouteMatchInterface $route_match) {
  switch ($route_name) {
    // Main module help for the metatag_custom_route module.
    case 'help.page.metatag_custom_route':
      $output = '';
      $output .= '<h3>' . t('About') . '</h3>';
      $output .= '<p>' . t('My Awesome Module') . '</p>';
      return $output;

    default:
  }
}

/**
 * Implements hook_page_attachments_alter().
 */
function metatag_custom_route_page_attachments_alter(array &$attachments) {
  $metatag_attachments = metatag_custom_route_get_metatag();
  if (!$metatag_attachments) {
    return;
  }
  // If any Metatag items were found, append them.
  if (!empty($metatag_attachments['#attached']['html_head'])) {
    if (empty($attachments['#attached'])) {
      $attachments['#attached'] = [];
    }
    if (empty($attachments['#attached']['html_head'])) {
      $attachments['#attached']['html_head'] = [];
    }
    foreach ($metatag_attachments['#attached']['html_head'] as $item) {
      $attachments['#attached']['html_head'][] = $item;
    }
  }

}

function metatag_custom_route_get_metatag() {
  $metatag_manager = \Drupal::service('metatag.manager');
  $route_match = \Drupal::routeMatch();
  $route_name = $route_match->getRouteName();
  //dpm($route_name);
  // First, get defaults.
  $metatags = metatag_get_default_tags();

  // Get metatags of entity
  $query = \Drupal::entityQuery('metatag_route')
    ->condition('name', $route_name);
  $metatag_route_storage = \Drupal::entityManager()->getStorage('metatag_route');
  $mrids = $query->execute();

  if(!empty($mrids)) {
    $entity = $metatag_route_storage->load(reset($mrids));
  }else {
    return;
  }

  if (!empty($entity) && $entity instanceof MetatagRouteInterface) {
    foreach ($metatag_manager->tagsFromEntity($entity) as $tag => $data) {
      $metatags[$tag] = $data;
    }
  }
  return $metatag_manager->generateElements($metatags, $entity);
}