Use multiple threads to merge distinct_count result on server

[Jackie-Jiang]

No description provided.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 67.10526% with 25 lines in your changes missing coverage. Please review.

Project coverage is 63.41%. Comparing base (59551e4) to head (6ceaf44).
Report is 1753 commits behind head on master.

Files with missing lines	Patch %	Lines
...e/operator/combine/AggregationCombineOperator.java	67.12%	14 Missing and 10 partials ⚠️
...org/apache/pinot/core/common/ObjectSerDeUtils.java	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #15111      +/-   ##
============================================
+ Coverage     61.75%   63.41%   +1.66%     
- Complexity      207     1482    +1275     
============================================
  Files          2436     2745     +309     
  Lines        133233   154384   +21151     
  Branches      20636    23823    +3187     
============================================
+ Hits          82274    97902   +15628     
- Misses        44911    49080    +4169     
- Partials       6048     7402    +1354     

Flag	Coverage Δ
custom-integration1	100.00% <ø> (+99.99%)	⬆️
integration	100.00% <ø> (+99.99%)	⬆️
integration1	100.00% <ø> (+99.99%)	⬆️
integration2	0.00% <ø> (ø)
java-11	63.39% <67.10%> (+1.68%)	⬆️
java-21	63.31% <67.10%> (+1.68%)	⬆️
skip-bytebuffers-false	63.41% <67.10%> (+1.66%)	⬆️
skip-bytebuffers-true	63.29% <67.10%> (+35.56%)	⬆️
temurin	63.41% <67.10%> (+1.66%)	⬆️
unittests	63.41% <67.10%> (+1.66%)	⬆️
unittests1	56.05% <67.10%> (+9.16%)	⬆️
unittests2	33.93% <0.00%> (+6.20%)	⬆️

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