metrics: decide dashboard coverage for Kafka producer response rate

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What to build

Decide how TiCDC should expose and consume ticdc_sink_kafka_producer_response_rate. The metric is currently defined, registered, periodically updated from Sarama broker metrics, and cleaned up when the collector stops, but it is not referenced by the Classic, NextGen, or with-keyspace-name Grafana dashboards.

If response rate provides independent operational value for diagnosing Kafka broker communication, add consistent dashboard coverage. If it does not provide useful information beyond the existing request-rate and in-flight-request signals, remove the unused metric and its collection cost.

Acceptance criteria

• The operational value of Kafka producer response rate is explicitly decided and documented in the change.
• If retained, Classic, NextGen, and with-keyspace-name dashboards expose the metric with filters and grouping consistent with the other broker-level Kafka producer panels.
• If removed, its definition, registration, collector update, cleanup, and generated dashboard references are absent.
• Relevant Kafka metrics tests and dashboard generation checks pass.

Blocked by

None - can start immediately.