fix: [2.5] Fix index meta mutex contention

[bigsheeper]

issue: #37630

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

pr: #38775

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: bigsheeper
To complete the pull request process, please assign jiaoew1991 after the PR has been reviewed.
You can assign the PR to them by writing /assign @jiaoew1991 in a comment when ready.

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• internal/datacoord/OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.06%. Comparing base (147c6b0) to head (9555ca9).
Report is 2 commits behind head on 2.5.

Additional details and impacted files

@@           Coverage Diff           @@
##              2.5   #38778   +/-   ##
=======================================
  Coverage   81.05%   81.06%           
=======================================
  Files        1381     1381           
  Lines      195185   195178    -7     
=======================================
+ Hits       158207   158214    +7     
+ Misses      31411    31394   -17     
- Partials     5567     5570    +3     

Components	Coverage Δ
Client	78.26% <ø> (ø)
Core	69.34% <ø> (ø)
Go	83.04% <100.00%> (+<0.01%)	⬆️

Files with missing lines	Coverage Δ
internal/datacoord/index_meta.go	96.37% <100.00%> (+1.04%)	⬆️

... and 29 files with indirect coverage changes