enhance: decrease CPU overhead when calculating index file size

[jaime0815]

issue: #36578

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.34%. Comparing base (c056620) to head (e75997c).
Report is 10 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #36579      +/-   ##
==========================================
- Coverage   82.32%   73.34%   -8.98%     
==========================================
  Files        1279     1281       +2     
  Lines      151260   151436     +176     
==========================================
- Hits       124519   111077   -13442     
- Misses      21737    35333   +13596     
- Partials     5004     5026      +22     

Files with missing lines	Coverage Δ
internal/datacoord/index_meta.go	95.83% <100.00%> (+0.08%)	⬆️
internal/datacoord/meta.go	89.74% <100.00%> (-0.07%)	⬇️

... and 266 files with indirect coverage changes