ProduceRequest#partitionSizes() is not an atomic operation

Encountered a concurrency issue in method ProduceRequest#partitionSizes() while developing with Kafka. When both Thread 1 and Thread 2 concurrently call method ProduceRequest#partitionSizes(), Thread 2 may receive an incomplete or empty result if Thread 1 is still in the process of initializing partitionSizes. This is an incorrect state. the code to ensure that Thread 2 obtains the final state rather than an intermediate one.