enhance: [2.4] Add balance report log for qc balancer

[congqixia]

Cherry pick from master
pr: #36747
Related to #36746

[sre-ci-robot]

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[codecov]

Codecov Report

Attention: Patch coverage is 90.55118% with 12 lines in your changes missing coverage. Please review.

Project coverage is 71.96%. Comparing base (c69af44) to head (5d2e976).
Report is 2 commits behind head on 2.4.

Files with missing lines	Patch %	Lines
...ernal/querycoordv2/balance/multi_target_balance.go	0.00%	10 Missing ⚠️
internal/querycoordv2/balance/report.go	97.43%	1 Missing ⚠️
...ernal/querycoordv2/balance/score_based_balancer.go	97.95%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              2.4   #36749      +/-   ##
==========================================
+ Coverage   71.95%   71.96%   +0.01%     
==========================================
  Files        1064     1065       +1     
  Lines      140580   140673      +93     
==========================================
+ Hits       101159   101241      +82     
- Misses      35093    35095       +2     
- Partials     4328     4337       +9     

Files with missing lines	Coverage Δ
internal/querycoordv2/balance/balance.go	94.20% <100.00%> (ø)
...erycoordv2/balance/channel_level_score_balancer.go	95.17% <100.00%> (+0.24%)	⬆️
...al/querycoordv2/balance/rowcount_based_balancer.go	95.27% <100.00%> (+0.16%)	⬆️
internal/querycoordv2/balance/utils.go	48.00% <100.00%> (ø)
internal/querycoordv2/balance/report.go	97.43% <97.43%> (ø)
...ernal/querycoordv2/balance/score_based_balancer.go	97.39% <97.95%> (-0.09%)	⬇️
...ernal/querycoordv2/balance/multi_target_balance.go	83.89% <0.00%> (-1.83%)	⬇️

... and 30 files with indirect coverage changes

[weiliu1031]

/lgtm