enhance: add mem size for index file and fallback to multiply with serialized size

[chyezh]

issue: #38715

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.08%. Comparing base (118678b) to head (098e487).
Report is 4 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #38716       +/-   ##
===========================================
+ Coverage   69.33%   81.08%   +11.74%     
===========================================
  Files         292     1381     +1089     
  Lines       26187   195256   +169069     
===========================================
+ Hits        18158   158331   +140173     
- Misses       8029    31334    +23305     
- Partials        0     5591     +5591     

Components	Coverage Δ
Client	78.26% <ø> (∅)
Core	69.33% <ø> (ø)
Go	83.07% <100.00%> (∅)

Files with missing lines	Coverage Δ
internal/querynodev2/segments/segment.go	64.30% <100.00%> (ø)
pkg/util/paramtable/component_param.go	98.38% <100.00%> (ø)

... and 1087 files with indirect coverage changes

[tedxu]

/lgtm

[xiaofan-luan]

/lgtm
/approve

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chyezh, xiaofan-luan

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• configs/OWNERS [xiaofan-luan]
• internal/querynodev2/segments/OWNERS [xiaofan-luan]
• pkg/util/OWNERS [xiaofan-luan]

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