enhance: Optimize DescribeIndex to reduce lock contention

[xiaocai2333]

issue: #29313
issue: #30443
master pr: #30939

[codecov]

Codecov Report

Attention: Patch coverage is 98.55072% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 82.20%. Comparing base (ef84d40) to head (013514d).
Report is 4 commits behind head on 2.3.

Additional details and impacted files

@@            Coverage Diff             @@
##              2.3   #30975      +/-   ##
==========================================
+ Coverage   82.18%   82.20%   +0.01%     
==========================================
  Files         843      843              
  Lines      121541   121573      +32     
==========================================
+ Hits        99892    99936      +44     
+ Misses      18421    18413       -8     
+ Partials     3228     3224       -4     

Files	Coverage Δ
internal/datacoord/meta.go	89.10% <100.00%> (+0.24%)	⬆️
internal/datacoord/index_service.go	94.87% <97.82%> (+0.08%)	⬆️

... and 13 files with indirect coverage changes

[czs007]

/approve
/lgtm

[sre-ci-robot]

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