@Override
    public OptionalLong currentLag(TopicPartition topicPartition) {
        return delegate.currentLag(topicPartition);
    }
Get the consumer's current lag on the partition. Returns an "empty" {@link OptionalLong} if the lag is not known,
for example if there is no position yet, or if the end offset is not known yet.

<p>
This method uses locally cached metadata. If the log end offset is not known yet, it triggers a request to fetch
the log end offset, but returns immediately.

@param topicPartition The partition to get the lag for.

@return This {@code Consumer} instance's current lag for the given partition.

@throws IllegalStateException if the {@code topicPartition} is not assigned
clients/src/main/java/org/apache/kafka/clients/consumer/KafkaConsumer.java