How to use the prom-client.register.metrics function in prom-client

.createServer((req, res) => {
                console.log(`[Metrics] [info]: ${req.url} was requested.`);
                if (url.parse(req.url) === '/metrics') {
                    res.writeHead(200, { 'Content-Type': register.contentType });
                    res.write(register.metrics());
                    res.end();
                }
            }).listen(5590);
    }

server = http.createServer((req, res) => {
        if (req.method !== 'GET'
                || parseURL(req.url).pathname !== prometheusConfig.getPath()) {
            res.writeHead(400, { 'Content-Type': 'text/plain' });
            res.end('Bad Request');
            return;
        }

        res.writeHead(200, {
            'Content-Type': register.contentType
        });
        res.end(register.metrics());
    });

router.get("/metrics", noCacheMiddleware, (req, res) => {
      res.set("Content-Type", register.contentType);
      res.end(register.metrics());
    });
    logger.info({ path: "/metrics" }, "mounting metrics path on app");

server.get('/metrics', (req, res) => {
  res.status(200);
  res.set('Content-Type', register.contentType);
  res.end(register.metrics());
});

App.get('/metrics', (req, res) => {
        res.set('Content-Type', Register.contentType);
        res.end(Register.metrics());
    });
};

this.get('/prometheus', (req, res) => {
                res.set('Content-Type', prometheusRegister.contentType);
                res.end(prometheusRegister.metrics());
            });
        }