Metatag.php

<?php

namespace Drupal\sd8_migration\Plugin\migrate\process;

use Drupal\migrate\MigrateExecutableInterface;
use Drupal\migrate\ProcessPluginBase;
use Drupal\migrate\Row;

/**
 * @MigrateProcessPlugin(
 *   id = "sd8_metatag"
 * )
 */
class Metatag extends ProcessPluginBase {

  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
    if ($value = @unserialize($value)) {
      foreach ($value as $name => $data) {
        if (!empty($data['value'])) {
          $metatag[$this->configuration['map'][$name]] = $data['value'];
        }
      }
    }
    return !empty($metatag) ? serialize($metatag) : NULL;
  }

}

migrationdefinition.php

    $tags = \Drupal::service('plugin.manager.metatag.tag')->getDefinitions();
    $definition['process']['meta_tags'] = [
      'plugin' => 'sd8_metatag',
      'source' => 'metatag',
      'map' => array_column($tags, 'id', 'name'),
    ];

module.php

/**
 * Implements hook_migrate_prepare_row().
 */
function sd8_migration_migrate_prepare_row(Row $row, MigrateSourceInterface $source, MigrationInterface $migration) {
  if ($source instanceof Node) {
    /** @var \Drupal\Core\Database\Query\SelectInterface $query */
    $query = $source->getDatabase()->select('metatag', 'm')
      ->fields('m', ['data'])
      ->condition('entity_type', 'node')
      ->condition('entity_id', $row->getSourceProperty('nid'))
      ->condition('revision_id', $row->getSourceProperty('vid'))
      ->condition('language', $row->getSourceProperty('language'));
    $row->setSourceProperty('metatag', $query->execute()->fetchField());
  }
}