On incoming requests, we should pick up the sentry-trace header and derive the transaction from it.
Like here in Laravel
https://github.com/getsentry/sentry-laravel/blob/ee3434f62981faabb1803e5ed57e76bc843dca02/src/Sentry/Laravel/Tracing/Middleware.php#L87

If we don't do this, we can't connect frontend -> backend.