Override

import { EventEmitter } from 'events';

class MetricsBus extends EventEmitter {
  emitted: number = 0;
  attached: number = 0;

  // Wrap emit to count every dispatch, then delegate to the real one.
  emit(event: string, name: string, value: number): boolean {
    this.emitted++;
    return super.emit(event, name, value);
  }

  // Wrap on to count subscriptions; stays chainable via super.on's `this`.
  on(event: string | symbol, listener: (...args: any[]) => void): this {
    this.attached++;
    return super.on(event, listener);
  }
}

const bus = new MetricsBus();
bus
  .on("metric", (name, value) => console.log("metric " + name + " = " + value))
  .on("metric", (name, value) => console.log("  (mirror) " + name));

bus.emit("metric", "cpu", 42);
bus.emit("metric", "mem", 71);

console.log("emitted: " + bus.emitted);
console.log("attached: " + bus.attached);
console.log("listeners: " + bus.listenerCount("metric"));