How were the records that were not processed (went to the DLQ) tracked?

Dead Letter Queue (DLQ) is used for storing messages that could not be processed. To track such records, the following approaches are usually applied:

• Logging errors during message processing with the reason for failure.
• Monitoring the DLQ with specialized tools or dashboards that show the number and details of messages.
• Setting up alerts (notifications) when new messages appear in the DLQ.

For example, in a system based on MQTT or Kafka, you can set up a subscription to the DLQ and automatically collect statistics or trigger reprocessing.