KIP-913 — Support decreasing send's block time without worrying about metadata's fetch

Discussion Producer

KIP-913 proposes adding a `KafkaProducer.getCluster(String topic, long maxBlockTimeMs)` public method that triggers an immediate metadata fetch and returns the resulting `Cluster` object, allowing applications to warm up the metadata cache before handling their first request. On cold start, the first `KafkaProducer.send()` incurs the full metadata-fetch latency because the local cache is empty; pre-fetching metadata during application initialization moves this cost out of the critical path.

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