lib/metrics-server.ts

import express from 'express';
import PromClient from 'prom-client';

/**
 * Will launch the Prometheus metrics server
 *
 * @param serverPort - The listening port to bind into this metrics server.
 * @param hostname - The TCP host to attach the listener.
 * @returns void
 */
export function setupMetricsServer(serverPort: number, hostname: string): void {
    PromClient.collectDefaultMetrics();
    const metricsServer = express();

    metricsServer.get('/metrics', (req, res) => {
      PromClient.register.metrics()
        .then(metrics => { res.header('Content-Type', PromClient.register.contentType).send(metrics); })
        .catch(err => res.status(500).send(err));
    });

    metricsServer.listen(serverPort, hostname);
}