code: public void close() {
          boolean isPreviouslyClosed = isClosed.getAndSet(true);
          if (isPreviouslyClosed) {
              return;
          }
  
          AdminClient client;
          while ((client = clientCache.poll()) != null) {
              client.close();
          }
      }
docs: close the AdminPool, if no long required.
  After closed, all public methods will throw IllegalStateException