Report delta StringSet counters for streaming

[rohitsinha54]

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 recovery leading to large growth in cns file sizes.

Tested that now we only see deltas

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

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[rohitsinha54]

R: @robertwb

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[rohitsinha54]

Currently working on fixing the unit test.

[rohitsinha54]

Ran testRequestKeyedData_sendOnShutdownStreamThrowsWindmillStreamShutdownException locally and it passes.

[rohitsinha54]

@robertwb Can you have a look? Thanks.