fix: Fix index meta mutex contention

[bigsheeper]

issue: #37630

Reduce the frequency of updateIndexTasksMetrics to avoid holding the mutex for long periods.

[sre-ci-robot]

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[mergify]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.10%. Comparing base (94955e5) to head (55e83b4).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #38775      +/-   ##
==========================================
- Coverage   82.90%   81.10%   -1.81%     
==========================================
  Files        1089     1383     +294     
  Lines      169219   195625   +26406     
==========================================
+ Hits       140296   158653   +18357     
- Misses      23332    31381    +8049     
  Partials     5591     5591              

Components	Coverage Δ
Client	78.26% <ø> (ø)
Core	69.43% <ø> (∅)
Go	83.09% <100.00%> (+0.01%)	⬆️

Files with missing lines	Coverage Δ
internal/datacoord/index_meta.go	96.73% <100.00%> (+1.41%)	⬆️

... and 320 files with indirect coverage changes