public ThreadState plus(ThreadState other) {
      return new ThreadState("<multiple threads>",
        cpuTime.plus(other.cpuTime),
        userTime.plus(other.userTime),
        waiting.plus(other.waiting),
        blocking.plus(other.blocking),
        -1
      );
    }
Adds up the timers in two {@code ThreadState} instances
planetiler-core/src/main/java/com/onthegomap/planetiler/stats/ProcessInfo.java