code: static int addTracingStatistic(TracingStatistic tracingStatistic) {
      // Check to see if we can enable the tracing statistic before actually
      // adding it.
      if (tracingStatistic.enable()) {
        // No synchronization needed, since this is a copy-on-write array.
        extraTracingStatistics.add(tracingStatistic);
        // 99.9% of the time, this will be O(1) and return
        // extraTracingStatistics.length - 1
        return extraTracingStatistics.lastIndexOf(tracingStatistic);
      } else {
        return -1;
      }
    }
docs: Adds a new tracing statistic to a trace
  
  @param tracingStatistic to enable a run
  @return The index of this statistic (for use with stat.extraInfo()), or
  -1 if the statistic is not enabled.