enhance: add l0 metric and fix datacoord no need drop l0 segment when flush

[aoiasd]

relate: #27675

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[codecov]

Codecov Report

Merging #28373 (4c18652) into master (7f46f4c) will increase coverage by 0.02%.
Report is 43 commits behind head on master.
The diff coverage is 90.90%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #28373      +/-   ##
==========================================
+ Coverage   81.86%   81.88%   +0.02%     
==========================================
  Files         843      843              
  Lines      117770   117803      +33     
==========================================
+ Hits        96412    96463      +51     
+ Misses      18126    18111      -15     
+ Partials     3232     3229       -3     

Files	Coverage Δ
internal/datacoord/meta.go	87.19% <100.00%> (+0.03%)	⬆️
internal/datacoord/util.go	99.25% <100.00%> (+0.04%)	⬆️
pkg/metrics/datacoord_metrics.go	100.00% <100.00%> (ø)
pkg/metrics/metrics.go	100.00% <ø> (ø)
internal/datacoord/services.go	84.14% <50.00%> (-0.25%)	⬇️

... and 27 files with indirect coverage changes

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/run-cpu-e2e

[XuanYang-cn]

/lgtm

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/run-cpu-e2e

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/run-cpu-e2e

[congqixia]

/approve

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