code: public void recordCheckoutTimeUs(SocketDestination dest, long checkoutTimeUs) {
          if(dest != null) {
              getOrCreateNodeStats(dest).recordCheckoutTimeUs(null, checkoutTimeUs);
              recordCheckoutTimeUs(null, checkoutTimeUs);
          } else {
              this.checkoutTimeRequestCounter.addRequest(checkoutTimeUs * Time.NS_PER_US);
          }
      }
docs: Record the checkout wait time in us
  
  @param dest Destination of the socket to checkout. Will actually record
  if null. Otherwise will call this on self and corresponding child
  with this param null.
  @param checkoutTimeUs The number of us to wait before getting a socket