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.
src/main/java/com/google/maps/internal/ratelimiter/Stopwatch.java