`kafka_event_aggregator`: improve metrics

- Prefix to `kafka_event_aggregator` (rather than `aggregator`)
- Expose better metrics about Kafka's internal queue sizes

Start by locating the kafka_event_aggregator implementation and its existing metric definitions. Check how current metrics are named and where Kafka internal queue sizes can be read; done means the metrics use the kafka_event_aggregator prefix and expose the requested queue-size information.

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