public function serialize(SpanContext $context)
    {
        $ret = '00-' . $context->traceId();
        if ($context->spanId()) {
            $ret .= '-' . str_pad($context->spanId(), 16, "0", STR_PAD_LEFT);
        }
        if ($context->enabled() !== null) {
            $ret .= '-' . ($context->enabled() ? '01' : '00');
        }
        return $ret;
    }