enhance: Add back compactionTaskNum metrics

[XuanYang-cn]

Fix L0 compaction task recover unable to set segment not isCompacting

See also: #34460

[codecov]

Codecov Report

Attention: Patch coverage is 69.90291% with 31 lines in your changes missing coverage. Please review.

Project coverage is 80.66%. Comparing base (3c70463) to head (6bbcdd6).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #34583      +/-   ##
==========================================
+ Coverage   71.48%   80.66%   +9.18%     
==========================================
  Files        1127     1127              
  Lines      139271   139336      +65     
==========================================
+ Hits        99556   112400   +12844     
+ Misses      35426    22642   -12784     
- Partials     4289     4294       +5     

Files	Coverage Δ
internal/datacoord/compaction_policy_l0.go	97.80% <100.00%> (-0.05%)	⬇️
internal/datacoord/meta.go	86.16% <100.00%> (+0.17%)	⬆️
internal/metastore/kv/datacoord/kv_catalog.go	75.13% <100.00%> (ø)
internal/datacoord/compaction_task_clustering.go	47.74% <0.00%> (+0.40%)	⬆️
internal/datacoord/compaction_task_l0.go	63.33% <33.33%> (-1.40%)	⬇️
internal/datacoord/compaction_trigger_v2.go	72.57% <36.36%> (+0.31%)	⬆️
internal/datacoord/compaction_task_mix.go	66.97% <42.85%> (-2.61%)	⬇️
internal/datacoord/compaction.go	70.93% <84.74%> (-0.31%)	⬇️

... and 227 files with indirect coverage changes

[czs007]

/approve
/lgtm

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