metrics.js

const express = require('express');
const next = require('next');
const client = require('prom-client'); // Metrics library

const dev = process.env.NODE_ENV !== 'production';
const app = next({ dev });
const handle = app.getRequestHandler();

const port = process.env.PORT || 3000;

app.prepare().then(() => {
  const server = express();

  // Metrics route
  const collectDefaultMetrics = client.collectDefaultMetrics;
  collectDefaultMetrics();

  server.get('/metrics', async (req, res) => {
    try {
      res.set('Content-Type', client.register.contentType);
      res.end(await client.register.metrics());
    } catch (ex) {
      res.status(500).end(ex.message);
    }
  });

  // All other routes handled by Next.js
  server.all('*', (req, res) => {
    return handle(req, res);
  });

  server.listen(port, (err) => {
    if (err) throw err;
    console.log(`> Ready on http://localhost:${port}`);
  });
});