Support clustering mode

[Yeonju Hwang]

I can add multiple worker but it expose only one worker metrics at a time.
How can I support the cluster mode using this plugin?

[Yeonju Hwang]

I checked out the code, and find out that this library doesn't support AggregatorRegistry which is for clustering.
Who looking for the way can follow the steps below. It works.

1. Use createPrometheusExporterPlugin with each new registry and registry to AggregatorRegistry

• In worker,

const registry = new Registry();

createPrometheusExporterPlugin({ ..., register: registry, defaultMetrics: true, defaultMetricsOption: { register: registry }})
AggregatorRegistry.setRegistry(registry)

2. In master, I opened a new metrics server to expose aggregatorRegistry.

const registry = new AggregatorRegistry();
const metricsApp = express()
metricsApp.get('/metrics', async (req, res) => {
  try {
	  const metrics = await aggregatorRegistry.clusterMetrics();
	  res.set('Content-Type', aggregatorRegistry.contentType);
	  res.send(metrics);
  } catch (ex) {
	  res.status(500);
	  res.send(ex.message);
  }
  res.end()
});

metricsApp.listen({another port})

https://github.com/siimon/prom-client/tree/master#usage-with-nodejss-cluster-module