docs: private synchronized void returnTransport(Transport returning) {
          long now = System.currentTimeMillis();
          PooledTransport unwrapped;
  
          // check if they're returning a leased transport
          if (returning instanceof LeasedTransport) {
              LeasedTransport leasedTransport = (LeasedTransport) returning;
              unwrapped = leasedTransport.delegate;
          } else if (returning instanceof PooledTransport) {
              unwrapped = (PooledTransport) returning;
          } else {
              throw new EsHadoopIllegalStateException("Cannot return a non-poolable Transport to the pool");
          }
  
          // make sure that this is even a leased transport before returning it
          if (leased.containsKey(unwrapped)) {
              leased.remove(unwrapped);
              idle.put(unwrapped, now);
          } else {
              throw new EsHadoopIllegalStateException("Cannot return a Transport object to a pool that was not sourced from the pool");
          }
      }