code: public Stopwatch stop() {
      long tick = ticker.read();
      checkState(isRunning, "This stopwatch is already stopped.");
      isRunning = false;
      elapsedNanos += tick - startTick;
      return this;
    }
docs: Stops the stopwatch. Future reads will return the fixed duration that had elapsed up to this
  point.
  
  @return this {@code Stopwatch} instance
  @throws IllegalStateException if the stopwatch is already stopped.