docs: public AdminClient checkout() {
          if (isClosed.get()) {
              throw new IllegalStateException("Pool is closing");
          }
  
          AdminClient client;
  
          // Try to get one from the Cache.
          while ((client = clientCache.poll()) != null) {
              if (!client.isClusterModified()) {
                  return client;
              } else {
                  // Cluster is Modified, after the AdminClient is created. Close it
                  client.close();
              }
          }
  
          // None is available, create new one.
          return createAdminClient();
      }