A suggestion for the description field for the _permissions function.

It is not totally clear what Edit Meta Tags means.
So if

/**
 * Implements hook_permisson().
 */
function metatag_permission() {
  $permissions['administer meta tags'] = array(
    'title' => t('Administer meta tags.'),
    'restrict access' => TRUE,
  );
  $permissions['edit meta tags'] = array(
    'title' => t('Edit meta tags.'),
  );
  return $permissions;
}

was changed to below or something like it (minus the asterisks) it may make it easier to quickly grasp that setting?

/**
 * Implements hook_permisson().
 */
function metatag_permission() {
  $permissions['administer meta tags'] = array(
    'title' => t('Administer meta tags.'),
    'restrict access' => TRUE,
  );
  $permissions['edit meta tags'] = array(
    'title' => t('Edit meta tags ****per Entity creation ie Node, Tag etc.****'),
  );
  return $permissions;
}