fix: entry span url in endponts using Express middleware/router objects

[BFergerson]

The following code:

const app = express();
const testRouter = express.Router();

app.use('/test', testRouter);
testRouter.get('/express', (req, res) => {
    res.sendStatus(200);
});

app.listen(3000);

Results in entry spans with the URL of /express and not the correct URL of /test/express.

This PR solves that using req.originalUrl when available.

[kezhenxu94]

Nice catch!