Optimize Adaptive Server Selection

[praveenc7]

Summary

This PR enhances the server selection logic in the adaptive server selector by using a HashMap for efficient server rank lookups and a HashSet to track selected servers. These changes improve performance when processing large sets of segments and servers, resulting in faster and more scalable server selection.

Description

The existing implementation of the adaptive server selector uses a linear search on a list (serverRankList) to determine server rankings, leading to performance inefficiencies when handling large numbers of servers

• The serverRankMap is now used for ranking lookups, providing O(1) access to server ranks compared to the previous O(n) list iteration.
• A HashSet (selectedServers) is introduced to track which servers have been selected and use it exit early

Complexity:
Total complexity of the adaptive server selection process per segment is reduced to O(m) per segment (with m being the number of candidates), improving the overall process to O(n * m) for n segments.

Previous: O(n* m * s) (with s being the number of total servers).

[vvivekiyer]

Please attach the CPU flame graph where we saw the increased CPU utilization.
Also would be good to post the before vs after CPU utilization.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.

Project coverage is 57.92%. Comparing base (59551e4) to head (dad3a28).
Report is 1020 commits behind head on master.

Files with missing lines	Patch %	Lines
...instanceselector/ReplicaGroupInstanceSelector.java	92.85%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #13952      +/-   ##
============================================
- Coverage     61.75%   57.92%   -3.83%     
- Complexity      207      219      +12     
============================================
  Files          2436     2612     +176     
  Lines        133233   143201    +9968     
  Branches      20636    21985    +1349     
============================================
+ Hits          82274    82953     +679     
- Misses        44911    53752    +8841     
- Partials       6048     6496     +448     

Flag	Coverage Δ
custom-integration1	<0.01% <0.00%> (-0.01%)	⬇️
integration	<0.01% <0.00%> (-0.01%)	⬇️
integration1	<0.01% <0.00%> (-0.01%)	⬇️
integration2	0.00% <0.00%> (ø)
java-11	57.89% <92.85%> (-3.82%)	⬇️
java-21	57.80% <92.85%> (-3.83%)	⬇️
skip-bytebuffers-false	57.91% <92.85%> (-3.84%)	⬇️
skip-bytebuffers-true	57.76% <92.85%> (+30.03%)	⬆️
temurin	57.92% <92.85%> (-3.83%)	⬇️
unittests	57.92% <92.85%> (-3.83%)	⬇️
unittests1	40.74% <ø> (-6.15%)	⬇️
unittests2	27.98% <92.85%> (+0.25%)	⬆️

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[jasperjiaguo]

lgtm otherwise, linter is failing, could you please fix?

[vvivekiyer]

LGTM. Thanks for this optimization.

[jasperjiaguo]

minor. Thanks for the optimizations