Kafka settlement is offset-based.

When reporting settlement, we'd want to include messaging.kafka.message.offset on the settlement span as shown in #1155. But that span does not have to be reported per message (it can be done in the background periodically, for each 1000th message, etc).

It looks weird to have the offset attribute in the message namespace as it's not just a message property, but also a thing on its own. Someone may also want to create a metric for the offset (e.g. latest published offset vs latest consumed offset would show the size of the queue). In this case it's rather a property of the topic/consumer group.

Suggesting messaging.kafka.offset or even a more generic one messaging.offset since offset concept is popular in messaging world.