Change a metatag dynamicly

Add meta tag from controller

Example: Add <meta http-equiv="refresh" content="30">
$data = [
  '#tag' => 'meta',
  '#attributes' => [
    'http-equiv' => 'refresh',
    'content' => '30',
  ],
];
$output['#attached']['html_head'][] = [$data, 'refresh'];

And also:

The metatag module provide an API (hook) to alter all meta tags : hook_metatag_metatags_view_alter

Example:

NOTE : This is not the drupal best practice.

/**
 *
 * Implement hook_metatag_metatags_view_alter().
 *  - Translate 'description' metatag of the home page.
 * @param type $output
 * @param type $instance
 * @param type $options
 */
function mymodule_metatag_metatags_view_alter(&$output, $instance, $options) {
  if ($instance == 'global:frontpage') {
    // Use dpm or kint to get the field to change.    
    if (!empty($value = $output['description']['#attached']['drupal_add_html_head'][0][0]['#value'])) {
      $output['description']['#attached']['drupal_add_html_head'][0][0]['#value'] = "The new value";
    }
  }
}