code: function addListeners() {
  	if (listenersAdded) return;
  	listenersAdded = true;
  
  	if (cluster.isMaster) {
  		// Listen for worker responses to requests for local metrics
  		cluster.on('message', (worker, message) => {
  			if (message.type === GET_METRICS_RES) {
  				const request = requests.get(message.requestId);
  				message.metrics.forEach(registry => request.responses.push(registry));
  				request.pending--;
  
  				if (request.pending === 0) {
  					// finalize
  					requests.delete(message.requestId);
  					clearTimeout(request.errorTimeout);
  
  					if (request.failed) return; // Callback already run with Error.
  
  					const registry = AggregatorRegistry.aggregate(request.responses);
  					const promString = registry.metrics();
  					request.done(null, promString);
  				}
  			}
  		});
  	}
  }
docs: Adds event listeners for cluster aggregation. Idempotent (safe to call more
  than once).
  @return {void}