refactor: Optimize Kafka statistics helper to reduce allocations

- Eliminate 6 intermediate model classes (KafkaMetricsData, etc.);
  CreateMetricsDictionary works directly from deserialized KafkaStatistics
- Add PopulateMetricsDictionary for dictionary reuse across drain cycles
- Merge topic and partition iteration into single pass
- Remove duplicate client-level request-total/response-total metrics
- Replace LINQ batch-size averaging with running sum
- Use actual partition rxbytes for consumer topic bytes-consumed-total

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: tippmar-nr