Cleanup the segment metadata reading logic in SegmentPreProcessor

[Jackie-Jiang]

• Always use the one from SegmentDirectory to avoid bugs introduced by inconsistent segment metadata
• Reduce some unnecessary metadata load

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 87.50000% with 7 lines in your changes missing coverage. Please review.

Project coverage is 63.24%. Comparing base (1a476de) to head (699a41a).
Report is 52 commits behind head on master.

Files with missing lines	Patch %	Lines
...ocal/segment/index/loader/SegmentPreProcessor.java	87.50%	2 Missing and 5 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #15746      +/-   ##
============================================
+ Coverage     62.90%   63.24%   +0.33%     
- Complexity     1386     1388       +2     
============================================
  Files          2867     2887      +20     
  Lines        163354   165091    +1737     
  Branches      24952    25223     +271     
============================================
+ Hits         102755   104408    +1653     
+ Misses        52847    52814      -33     
- Partials       7752     7869     +117     

Flag	Coverage Δ
custom-integration1	100.00% <ø> (ø)
integration	100.00% <ø> (ø)
integration1	100.00% <ø> (ø)
integration2	0.00% <ø> (ø)
java-11	63.22% <87.50%> (+0.35%)	⬆️
java-21	63.19% <87.50%> (+0.36%)	⬆️
skip-bytebuffers-false	?
skip-bytebuffers-true	?
temurin	63.24% <87.50%> (+0.33%)	⬆️
unittests	63.23% <87.50%> (+0.33%)	⬆️
unittests1	56.31% <30.35%> (+0.49%)	⬆️
unittests2	33.47% <87.50%> (-0.11%)	⬇️

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