constructor(options) {
    const {numWorkers, reportCallback} = options;
    this.numWorkers = numWorkers;
    this.reportCallback = reportCallback;
    this.interval = null;
    this.reset();
  }
constructor
@param options {Object} - Monitor options.
@param options.numWorkers {number} - The number of workers.
@param options.reportCallback {function} - Callback called on each monitor report.