metrics: clarify ownership of the codec compression ratio metric

What to build

Clarify the ownership and operational use of ticdc_kafka_sink_codec_compression_ratio. The metric is defined in the codec package shared by Kafka and Pulsar, but its Prometheus family name is Kafka-specific. It has a producer and cleanup path, while the Classic, NextGen, and with-keyspace-name dashboards do not consume it.

Determine which sinks emit the metric and choose one coherent outcome: keep it as Kafka-owned, make it sink-neutral, add explicit sink identity, or remove it if it has no operational value. Keep the definition, producer, cleanup, registration, and dashboard behavior consistent with that decision.

Acceptance criteria

• The Kafka and Pulsar code paths that can emit the codec compression ratio are identified.
• The metric owner, family name, labels, and operational purpose are explicitly decided.
• Definition, registration, write, cleanup, and dashboard coverage are consistent with the chosen design.
• Any metric rename or removal accounts for affected dashboards, alerts, and recording rules.
• Relevant codec and sink tests plus dashboard generation checks pass.

Blocked by

None - can start immediately.