KIP-92 — Add per partition lag metrics to KafkaConsumer

Accepted ConsumerMetrics

KIP-92 adds per-partition consumer lag metrics (`records-lag`, `records-lag-avg`, `records-lag-max`) scoped by topic and partition to the `consumer-fetch-manager-metrics` group in `KafkaConsumer`. The existing metric only exposes the maximum lag across all assigned partitions, making it impossible to detect which specific partition is falling behind.

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