Issue while passing express app in the options

[iravishah]

this.request.proto = parent.request;
^

TypeError: Cyclic proto value
at IncomingMessage.set proto [as proto] ()

[joao-fontenele]

hello, please provide more details of how the error occurred, a minimal snippet of code, that reproduces the error would be very heplful

[iravishah]

`const express = require('express');
const httpCollector = require('express-prometheus-middleware');
const app = express();

const PORT = 9091;

app.use(httpCollector({
metricsPath: '/metrics',
collectDefaultMetrics: false,
collectGCMetrics: false,
metricsApp: app,
// requestDurationBuckets: [0.1, 0.5, 1, 1.5],
/**

• Uncomenting the authenticate callback will make the metricsPath route
• require authentication. This authentication callback can make a simple
• basic auth test, or even query a remote server to validate access.
• To access /metrics you could do:
• curl -X GET user:password@localhost:9091/metrics
  /
  // authenticate: req => req.headers.authorization === 'Basic dXNlcjpwYXNzd29yZA==',
  /*
• Uncommenting the extraMasks config will use the list of regexes to
• reformat URL path names and replace the values found with a placeholder value
  /
  // extraMasks: [/..:..:..:..:..:../],
  /*
• The prefix option will cause all metrics to have the given prefix.
• E.g.: app_prefix_http_requests_total
  /
  // prefix: 'test_',
  /*
• Can add custom labels with customLabels and transformLabels options
  */
  customLabels: ['contentType', 'statusCode'],
  transformLabels(labels, req, res) {
  // // eslint-disable-next-line no-param-reassign
  labels.contentType = req.headers['content-type'];
  labels.statusCode = res.statusCode;
  },
  }));

app.get('/test', (req, res, next) => {
res.status(400).send('ok')
});

app.listen(PORT, () => console.log(Example app listening on port ${PORT}!));`

Here in the snippet, I have used the main application's express app instance metricsApp: app.

[joao-fontenele]

Hello, this is an interesting case.

If you plan to use the same port and express app to respond the /metrics route, you don't need to pass an express instance. So a quick fix is just to not pass the metricsApp argument to the lib. To fix, I will likely have to document this behavior, or will find a way to check this specific case