MessagingMetricsProcessorServiceTest is flaky

As seen in https://builds.cask.co/browse/CDAP-BUT-1341
MessagingMetricsProcessorServiceTest#persistMetricsTests failed with

This is because in MessagingMetricsProcessorService, before persisting the metrics and topic metas, a copy of the topic metas containing the metrics processor delay metrics is made before making a copy of metric values. Therefore, there can be a very small chance where all metric values are persisted but the corresponding metric metas are not yet persisted.