Lazy-load Node.js Cluster module

[laurentgoudet]

Passenger provides its own clustering, and hence is not compatible with Node.js cluster module. In order to improve the developer experience, it provides a non-functional shim while warning developers about the underlying issue:

Trace: You required the Node Cluster module, which is incompatible with Passenger, a non-functional shim was returned and your app may still work. However, please remove the related code as soon as possible.
    at Module.require (/usr/share/passenger/helper-scripts/node-loader.js:63:12)
    at require (internal/modules/cjs/helpers.js:22:18)
    at Object.<anonymous> (/home/app/webapp/node_modules/prom-client/lib/cluster.js:11:17)
    at Module._compile (internal/modules/cjs/loader.js:689:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:700:10)
    at Module.load (internal/modules/cjs/loader.js:599:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:538:12)
    at Function.Module._load (internal/modules/cjs/loader.js:530:3)
    at Module.require (internal/modules/cjs/loader.js:637:17)
    at Module.require (/usr/share/passenger/helper-scripts/node-loader.js:80:25)

It would be nice if prom-client would lazy-load the Node.js cluster module until its aggregation features are used, i.e. until an AggregatorRegistry instance is created, in order to provide a smooth experience when integrating with Passenger.

For reference a similar issue was reported & fixed on log4js-node 2 years ago, although I don't think a disableClustering: true flag is needed here as one can simply not use the AggregatorRegistry in the first place when not using Node.js clustering.