make flink container metric cache threadsafe

[minxhe]

We have been observing intermittent missing metrics for Beam metrics emitted from async threads. This pull request improves thread safety in the Flink metrics implementation by switching to concurrent data structures. The most important changes are:

Flink metrics thread safety:

• Replaced HashMap with ConcurrentHashMap for flinkCounterCache, flinkDistributionGaugeCache, and flinkGaugeCache in FlinkMetricContainerBase.java to improve thread safety in metric storage. [1] [2]

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