给aggregate查询加limit行数限制，防止返回结果过多导致archery挂掉

[czxin788]

fix bug：#1946

[codecov]

Codecov Report

Base: 75.91% // Head: 75.90% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (17c3370) compared to base (3ff5c95).
Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1948      +/-   ##
==========================================
- Coverage   75.91%   75.90%   -0.01%     
==========================================
  Files          91       91              
  Lines       14203    14204       +1     
==========================================
  Hits        10782    10782              
- Misses       3421     3422       +1     

Impacted Files	Coverage Δ
sql/engines/mongo.py	49.79% <0.00%> (-0.07%)	⬇️

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[nick2wang]

重新跑了一遍CI都通过了，还有一个lint报错劳烦用black格式化下代码就可以了

[czxin788]

明白了，用black格式化以后，将'$limit'修改成"$limit"了，引号的问题