Andrii Babanin

Have you called the `Dispose` method? Kafka client spawns a background polling task to handle operation completion notifications. Calling the Dispose method should eventually [cancel this task](https://github.com/confluentinc/confluent-kafka-dotnet/blob/8678c00150340a6b6169ed766daf566cb3cd3864/src/Confluent.Kafka/AdminClient.cs#L1665).

+1 Having cancelable async operations is great - I wish the Kafka client had more methods like that. However, extending a method signature with a CancellationToken might break binary compatibility,...