enhance: [2.5]Add missing delete metrics (#38634)

[XuanYang-cn]

Add 2 counter metrics:

• Total delete entries from deltalog: milvus_datanode_compaction_delete_count
• Total missing deletes: milvus_datanode_compaction_missing_delete_count

See also: #34665
pr: #38634

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[mergify]

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[codecov]

Codecov Report

Attention: Patch coverage is 96.09375% with 5 lines in your changes missing coverage. Please review.

Project coverage is 81.06%. Comparing base (a4901ef) to head (2227af7).
Report is 1 commits behind head on 2.5.

Files with missing lines	Patch %	Lines
internal/datanode/compaction/merge_sort.go	90.32%	2 Missing and 1 partial ⚠️
...ternal/datanode/compaction/clustering_compactor.go	86.66%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              2.5   #38747      +/-   ##
==========================================
+ Coverage   81.04%   81.06%   +0.02%     
==========================================
  Files        1381     1381              
  Lines      195154   195195      +41     
==========================================
+ Hits       158164   158244      +80     
+ Misses      31422    31393      -29     
+ Partials     5568     5558      -10     

Components	Coverage Δ
Client	78.26% <ø> (ø)
Core	69.33% <ø> (ø)
Go	83.05% <96.09%> (+0.02%)	⬆️

Files with missing lines	Coverage Δ
internal/datanode/compaction/compactor_common.go	87.33% <100.00%> (+2.84%)	⬆️
internal/datanode/compaction/mix_compactor.go	73.76% <100.00%> (-0.49%)	⬇️
pkg/metrics/datanode_metrics.go	100.00% <100.00%> (ø)
...ternal/datanode/compaction/clustering_compactor.go	65.87% <86.66%> (+0.29%)	⬆️
internal/datanode/compaction/merge_sort.go	79.50% <90.32%> (+0.56%)	⬆️

... and 36 files with indirect coverage changes