Reset sketch combiner in AggregatorCombiner

[a2l007]

Fixes #7741
This PR fixes an issue where the AggregateCombiner.reset() within SketchAggregatorFactorydoes not clear the Sketch union object. As a result, during index merging if the sketches are empty for a set of rows to be combined, the combined sketch value from the previous set of merged rows would be reused in the current combine thereby corrupting the data.
This PR has:

• been self-reviewed.
• added unit tests or modified existing tests to cover new code paths.
• been tested in a test Druid cluster.

@leventov Please review.

[himanshug]

LGTM but build failure is genuine

[leventov]

For context, #7329 could be used to test AggregatorCombiner.reset() in all aggregators

[a2l007]

That's weird. I don't seem to hit this issue when I run the tests locally. Let me look into it.

[gianm]

coverage/coveralls — Coverage decreased (-2.8%) to 65.21%

Coveralls errored with a -2.8% coverage decrease. This seems incorrect: only one line of prod code was changed, and there is a test for it. So I'll merge the patch anyway.

[a2l007]

@clintropolis Would it be worth calling this patch out in the Release Notes? It would be useful for the datasketch users to know that existing theta sketch aggregates generated with releases since 0.13.0 may be invalid and would have to be recomputed in 0.16.0

[clintropolis]

@clintropolis Would it be worth calling this patch out in the Release Notes? It would be useful for the datasketch users to know that existing theta sketch aggregates generated with releases since 0.13.0 may be invalid and would have to be recomputed in 0.16.0

Yeah, I think so, I'll be sure to add mention of this, thanks!