KIP-223 — Add per-topic min lead and per-partition lead metrics to KafkaConsumer

Accepted Kafka 2.0 ConsumerMetrics

KIP-223 adds per-partition consumer lead metrics (`records-lead`, `records-lead-avg`, `records-lead-min`) and a global `records-lead-min` metric to `KafkaConsumer`'s `consumer-fetch-manager-metrics` group. Lag measures how far behind the high watermark a consumer is, but there was no metric for how far ahead of the log-start-offset the consumer is—a value near zero signals that the consumer is about to stall or lose data due to log retention.

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