Periodically send perworkermetric updates on the WorkerMessages api.

[JayajP]

If EnablePerWorkerMetrics is True, then StreamingDataflowWorker will periodically extract PerWorkerMetric updates and send them on the WorkerMessages API. This happens in StreamingDataflowWorker::sendWorkerMessage.

Currently, we only support sending BigQuerySink metrics on this channel.

Additionally, when we extract counter updates we will take note of counters that are zero-valued, counters that have been zero-valued for more than 5 minutes will be removed from the underlying metrics storage (the StreamingStepMetricsContainer::perWorkerCounters map). This allows us to create counters based on bigquery table ids, which can be dynamically set with dynamic destinations.

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