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";

    }

  }

}