Restrict Metatag field restrict cardinality

There's code in it that used to work, but it must have stopped working recently.

There are tests in comment module for this, will adapt

This is the code in question. It did work last Spring. :) But that was a number of D8 Betas ago.

/**
 * Implements hook_form_alter().
 */
function metatag_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  // Disable the cardinality option on the field storage settings form for
  // metatag fields.
  if ($form_id == 'field_ui_field_storage_edit_form') {
    if ($form['#field']->getType() == 'metatag') {
      $form['field_storage']['cardinality_container']['#prefix'] = t("Metatag fields must be singular.");
      $form['field_storage']['cardinality_container']['#disabled'] = TRUE;
      $form['field_storage']['cardinality_container']['cardinality']['#disabled'] = TRUE;
      $form['field_storage']['cardinality_container']['cardinality_number']['#disabled'] = TRUE;
      unset($form['field_storage']['cardinality_container']['cardinality_number']['#states']);
    }
  }
}

Looks like a few things have changed. Here's a patch that works with Beta 16. (Sorry, haven't gone to RC 1 just yet. :) )

Committed. Thanks Michelle!

without a test :( - should I add a new issue for a follow up? Or add test here?

Please add new issue for tests for existing changes, or just roll it into the main "add tests" issue.

@larowlan - Sorry. :( Learning to write tests is on my "to learn" list but I haven't gotten to it, yet.

Added test coverage in #2563637: Write tests for the 8.x-1.x functionality for this bug.