KIP-674 — API to Aggregate Metrics in Kafka Streams

Discussion StreamsMetrics

Adds a `KafkaStreamsMetricsAggregator` interface and a built-in aggregating `MetricsReporter` implementation that can roll up Kafka Streams task-level and thread-level metrics into application-level aggregates. Commercial monitoring services often impose limits on the number of metrics reported; a Kafka Streams application with many tasks/partitions can easily exceed these limits.

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