feat(kafka): add messaging.kafka.cluster.id to producer and consumer spans

Is your feature request related to a problem? Please describe.

When a service connects to multiple Kafka clusters, or when the same topic name exists across environments (dev/staging/prod), Kafka spans carry no information about which cluster a message came from. `messaging.kafka.cluster.id` is defined in the OTel messaging semantic conventions for Kafka but is not currently set by this instrumentation — making it impossible to distinguish spans by cluster from telemetry alone.

Describe the solution you'd like

Add `messaging.kafka.cluster.id` as a span attribute on both producer and consumer spans. The cluster ID is read from the client's own already-fetched `Metadata` object — no `AdminClient`, no extra broker connection, no additional security config. The attribute is emitted by default — no opt-in flag required — and cached per client instance after the first successful read.

Implementation handles Kafka 3.7+ correctly: `KafkaConsumer` became a thin delegate wrapper in 3.7 (KIP-848) and no longer holds `metadata` directly — the fix walks `KafkaConsumer.delegate → LegacyKafkaConsumer` to find the `metadata` field.

Cross-language effort

This feature is being added consistently across all four OTel language instrumentations:

• Java (this repo): #18978 — reads cluster id from already-fetched `Metadata`; no extra broker connection
• JavaScript: open-telemetry/opentelemetry-js-contrib#3577 — fetches via `admin().describeCluster()` once per lifecycle
• Python: open-telemetry/opentelemetry-python-contrib#4727 — reads from aiokafka/kafka-python/confluent-kafka client metadata
• .NET: open-telemetry/opentelemetry-dotnet-instrumentation#5206 — one-time background `AdminClient.DescribeClusterAsync()` per bootstrap-servers string

Describe alternatives you've considered

• Extracting from consumer `metrics()` tags — Kafka does not expose the cluster ID as a standard metric tag, so this is not reliable across versions.
• Requiring users to configure the cluster ID manually — eliminates the zero-config goal.

Additional context

`messaging.kafka.cluster.id` is defined in the OTel semconv incubating attributes (KIP-78). The attribute is emitted by default, consistent with the other `Recommended` Kafka attributes this instrumentation already produces. A working implementation with staging verification is ready.

Tip

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Review the Java implementation referenced in pull request #18978, focusing on the Kafka producer and consumer spans, KafkaConsumer.delegate, LegacyKafkaConsumer, and the already-fetched Metadata object. Confirm that the cluster ID is added to both span types without an extra broker connection and that it is cached per client instance.