code: public boolean complete() {
      Map<ClientTransport.PingCallback, Executor> callbacks;
      long roundTripTimeNanos;
      synchronized (this) {
        if (completed) {
          return false;
        }
        completed = true;
        roundTripTimeNanos = this.roundTripTimeNanos = stopwatch.elapsed(TimeUnit.NANOSECONDS);
        callbacks = this.callbacks;
        this.callbacks = null;
      }
      for (Map.Entry<ClientTransport.PingCallback, Executor> entry : callbacks.entrySet()) {
        doExecute(entry.getValue(), asRunnable(entry.getKey(), roundTripTimeNanos));
      }
      return true;
    }
docs: Completes this operation successfully. The stopwatch given during construction is used to
  measure the elapsed time. Registered callbacks are invoked and provided the measured elapsed
  time.
  
  @return true if the operation is marked as complete; false if it was already complete