refactor(flow): use atomic.Int64 in Throttler

[reugn]

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[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.86%. Comparing base (386f6d8) to head (1fce57d).

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #169      +/-   ##
==========================================
+ Coverage   99.59%   99.86%   +0.27%     
==========================================
  Files          13       13              
  Lines         738      738              
==========================================
+ Hits          735      737       +2     
+ Misses          3        1       -2     

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