enhance: decrease CPU overhead when calculating index file size

[jaime0815]

issue: #36578
pr: #36579

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.89%. Comparing base (e5a6c5b) to head (878767b).
Report is 1 commits behind head on 2.4.

Additional details and impacted files

@@            Coverage Diff             @@
##              2.4   #36580      +/-   ##
==========================================
- Coverage   71.91%   71.89%   -0.03%     
==========================================
  Files        1064     1064              
  Lines      140331   140335       +4     
==========================================
- Hits       100925   100894      -31     
- Misses      35089    35109      +20     
- Partials     4317     4332      +15     

Files with missing lines	Coverage Δ
internal/datacoord/index_meta.go	95.73% <100.00%> (+0.08%)	⬆️
internal/datacoord/meta.go	89.88% <100.00%> (-0.07%)	⬇️

... and 32 files with indirect coverage changes