Export aggregated metrics to prometheus using the cluster module

[gabrielgradinaru]

Is your feature request related to a problem? Please describe.
I can't find a way to export aggregated metrics to prometheus using Node.js's cluster module

Describe the solution you'd like

• I would like to be able to export aggregated metrics using opencensus-exporter-prometheus and Node.js's cluster module

• I would like some docs for this feature similar to: https://github.com/siimon/prom-client#usage-with-nodejss-cluster-module

Describe alternatives you've considered
I've tried to export metrics in each worker process, but this is undesirable.
I would like to be able to export metrics aggregated for the whole cluster.

[mayurkale22]

If you are feeling motivated, maybe you can attempt to write solution and open a PR.