OptionsKeyFormatter.php

<?php

namespace Drupal\options\Plugin\Field\FieldFormatter;

use Drupal\Core\Field\FieldFilteredMarkup;
use Drupal\Core\Field\FormatterBase;
use Drupal\Core\Field\FieldItemListInterface;

/**
 * Plugin implementation of the 'list_key' formatter.
 *
 * @FieldFormatter(
 *   id = "list_key",
 *   label = @Translation("Key"),
 *   field_types = {
 *     "list_integer",
 *     "list_float",
 *     "list_string",
 *   }
 * )
 */
class OptionsKeyFormatter extends FormatterBase {
    
    /**
     * {@inheritdoc}
     */
    public function viewElements(FieldItemListInterface $items, $langcode) {
        $elements = [];
        foreach ($items as $delta => $item) {
            $elements[$delta] = [
                '#markup' => $item->value,
                '#allowed_tags' => FieldFilteredMarkup::allowedTags(),
            ];
        }
        return $elements;
    }

}