enhance: add metrics for segment prune latnecy(#30376)

[MrPresent-Han]

related: #30376

[mergify]

@MrPresent-Han ut workflow job failed, comment rerun ut can trigger the job again.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.70%. Comparing base (ac28402) to head (dedd563).
Report is 56 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #34094       +/-   ##
===========================================
+ Coverage   69.80%   80.70%   +10.90%     
===========================================
  Files        1077     1077               
  Lines      137148   137156        +8     
===========================================
+ Hits        95731   110698    +14967     
+ Misses      37201    22239    -14962     
- Partials     4216     4219        +3     

Files	Coverage Δ
internal/querynodev2/delegator/segment_pruner.go	69.81% <100.00%> (+1.39%)	⬆️
pkg/metrics/querynode_metrics.go	100.00% <100.00%> (ø)

... and 263 files with indirect coverage changes

[MrPresent-Han]

rerun ut

[congqixia]

/lgtm

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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