_pushActivity(spanId) {
    this.cancelIdleTimeout(undefined, { restartOnChildSpanChange: !this._idleTimeoutCanceledPermanently });
    debugBuild.DEBUG_BUILD && utils.logger.log(`[Tracing] pushActivity: ${spanId}`);
    this.activities[spanId] = true;
    debugBuild.DEBUG_BUILD && utils.logger.log('[Tracing] new activities count', Object.keys(this.activities).length);
  }
Start tracking a specific activity.
@param spanId The span id that represents the activity