Response to question on https://retgits.github.io/monitoring-your-node-js-apps-with-prometheus/

index.js

var express = require('express');
var router = express.Router();
var url = require('url');

router.get('/api', function(req, res, next) {

    var err = "this is error";
    var data = { data: "Succuss"}
    if (err) {
        var error = Error (err)
        next(error.message)
        /**
         * I've added the below two lines to make the code work as you expected it (or at least how I think you wanted it to work :))
         * res.statusCode sets the HTTP Status code, in this case to 400, so Prometheus can use that to differentiate between the various codes
         * the original code would always result in statusCode 304.
         * To add the status message you can simply do 'res.message = err' (see below). This will create a new key in the res object that your Prometheus
         * method will use. 
         */
        res.statusCode = 400;
        res.message = err
        
        res.json({message:err})
    }else {
        res.json(data);
    }
	
});

module.exports = router;