public Stopwatch stop() {
		long tick = ticker.read();
		checkState(isRunning, "This stopwatch is already stopped.");
		isRunning = false;
		elapsedNanos += tick - startTick;
		return this;
	}
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.
shared/src/main/java/com/oasisfeng/perf/Stopwatch.java