feat: update to otel semconv 1.40.0

[epbensimpson]

Description

Updates KafkaFlow.OpenTelemetry to the OTEL Semantic Conventions 1.40.0 (current version at time of writing).

https://github.com/open-telemetry/semantic-conventions/blob/v1.40.0/docs/messaging/kafka.md

This contains some breaking changes to attribute names due to the unstable nature of the spec.

Implements #470

Summary of changes

Removed

Attribute	Reason
peer.service	Contained the bootstrap server list, which does not represent the broker that handled a given message

Renamed

Old	New	Where
messaging.operation = "publish"	messaging.operation.type = "send" + messaging.operation.name = "publish"	Producer
messaging.operation = "process"	messaging.operation.type = "receive" + messaging.operation.name = "process"	Consumer
messaging.source.name	messaging.destination.name	Consumer
messaging.kafka.consumer.group	messaging.consumer.group.name	Consumer
messaging.kafka.message.offset	messaging.kafka.offset	Both
messaging.kafka.source.partition	messaging.destination.partition.id	Consumer
messaging.kafka.destination.partition	messaging.destination.partition.id	Producer

Added

Attribute	Where	Notes
messaging.client.id	Consumer	Set to ConsumerContext.ConsumerName — closest available proxy for the Kafka client ID, which is not exposed by IConsumerContext
messaging.kafka.message.tombstone = true	Both	Only emitted when message value is null
messaging.message.body.size	Consumer	Only emitted when the message value is still byte[] (i.e. before deserialization middleware has run)
error.type	Both	Emitted in error handlers only; value is the exception's fully-qualified type name

Bug fix

messaging.kafka.message.key: the consumer previously always emitted this tag (empty string for null keys, with an unsafe cast to byte[]), and the producer emitted the raw
object. Both now share a FormatMessageKey helper that UTF-8 decodes byte[] keys, calls ToString() on other types, and omits the tag entirely when the key is null.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have added tests to cover my changes
• I have made corresponding changes to the documentation

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