Report delta stringset metrics for streaming rather than cumulative.
Reporting cumulative was causing the following issues:

• Every periodic (10 seconds) reports took cumulative over and over and reported it hence every report was reporting the metric. Unlike batch job reporting where it filters to only take one which has changed (tracked by dirty bit).
• Not reseting was using more memory as metrics remained in memory forever
• In backend it lead to large memory consumption when tracking active workitem counters which are also persisted for in level db by monitoring state store.

Tested that now we only see deltas

https://screenshot.googleplex.com/7W7R8Znv3mLgP8h
https://screenshot.googleplex.com/6GMLuRMe9zKrAMV

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