Fix: mongodb 用不了count统计

[fixpng]

pymongo4.x版本
Collection.count和Cursor.count已删除
已删除 pymongo.collection.Collection.count() 和 pymongo.cursor.Cursor.count() 。使用 count_documents() 或 estimated_document_count() 取而代之
fix #2982

[codecov]

Codecov Report

Attention: Patch coverage is 96.77419% with 1 line in your changes missing coverage. Please review.

Project coverage is 78.51%. Comparing base (c647f07) to head (d5ddbb1).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
sql/engines/mongo.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2991      +/-   ##
==========================================
+ Coverage   78.48%   78.51%   +0.03%     
==========================================
  Files         125      125              
  Lines       17792    17822      +30     
==========================================
+ Hits        13964    13993      +29     
- Misses       3828     3829       +1     

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