v2.5.9

• [Enhancement] Optimize StatisticsDecorator to eliminate per-hash Array allocations by using each_pair with a per-call pending-writes buffer instead of current.keys.each, reducing allocations from tens of thousands to one per call at scale.
• [Enhancement] Inline StatisticsDecorator#append and #suffix_keys_for into #diff to reduce method call overhead by ~96% (from ~915k to ~39k calls at 6400 partitions).
• [Enhancement] Support excluded_keys option in StatisticsDecorator to skip decoration of unused librdkafka statistics subtrees (e.g. broker toppars, window stats), yielding ~28% additional speedup at scale.