code: public long done() {
      if (enabled) {
        if (!done) {
          lastCheckpointNanos = System.nanoTime();
          done = true;
        }
        return lastCheckpointNanos - startNanos;
      }
      return -1;
    }
docs: Indicate we are done (stop the timer). Once you call this
  method, subsequent calls have no effect.
  
  @return time in nanoseconds from the start of this metric, or -1
  if {@code false} was passed in the constructor