Altera a saída do block image, para adicionar o custom_credit

gistfile1.txt

public function __construct()
  {
    add_filter('render_block', array($this, 'render_custom_image'), 10, 2);
  }

  public function render_custom_image($block_content, $block)
  {
    if ($block['blockName'] === 'core/image') {
      $image_id = $block['attrs']['id'] ?? null;

      if ($image_id) {
        $custom_credit = get_post_meta($image_id, 'custom_credit', true);

        if (!empty($custom_credit)) {
          $custom_credit_html = sprintf('<span class="custom-credit">%s %s</span>', __('Credit:', BOOTSTRAPTHEME_TEXTDOMAIN), esc_html($custom_credit));

          if (strpos($block_content, '<figcaption class="wp-element-caption">') !== false) {
            $block_content = str_replace('</figcaption>', ' ' . $custom_credit_html . '</figcaption>', $block_content);
          } else {
            $block_content .= '<figcaption class="wp-element-caption">' . $custom_credit_html . '</figcaption>';
          }
        }
      }
    }

    return $block_content;
  }