This will include item meta keys for all forms on your site.

add_filter('frm_include_meta_keys', '__return_true');

This example will allow you to include item meta keys for entries belonging to a single form.

add_filter('frm_include_meta_keys', 'include_meta_keys_for_single_form', 10, 2);
function include_meta_keys_for_single_form( $include_keys, $args ) {
  if ( $args['form_id'] == 364 ) { //replace 364 with Form ID
	  $include_key = true;
  }
  return $include_key;
}