private function recordEvent($key)
    {
        $success = null;
        apcu_inc($key, 1, $success);
        if ($success !== null && !$success) {
            // @TODO: Remove this fallback when we drop APCu 4.x support.
            // @codeCoverageIgnoreStart
            // Ignore coverage because (1) it's tested with other code and
            // (2) APCu 5.x does not use it.
            $success = apcu_store($key, 1);
            // @codeCoverageIgnoreEnd
        }
        return $success;
    }