public void loadProducerEntry(ProducerStateEntry entry) {
        long producerId = entry.producerId();
        addProducerId(producerId, entry);
        entry.currentTxnFirstOffset().ifPresent(offset -> ongoingTxns.put(offset, new TxnMetadata(producerId, offset)));
    }
Get an unmodifiable map of active producers.
storage/src/main/java/org/apache/kafka/storage/internals/log/ProducerStateManager.java