chore: set gw rate limits at event level

[cisse21]

Description

Tweaking the existing GW Rate Limiter to throttle based on the incoming events rather than requests

Linear Ticket

Fixes PIPE-475

Security

• The code changed/added as part of this pull request won't create any security issues with how the software is being used.

[codecov]

Codecov Report

Attention: 143 lines in your changes are missing coverage. Please review.

Comparison is base (b80d273) 71.53% compared to head (8d7068f) 71.85%.
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4069      +/-   ##
==========================================
+ Coverage   71.53%   71.85%   +0.31%     
==========================================
  Files         373      373              
  Lines       54826    54905      +79     
==========================================
+ Hits        39219    39451     +232     
+ Misses      13272    13136     -136     
+ Partials     2335     2318      -17     

Files	Coverage Δ
gateway/throttler/throttler.go	80.28% <100.00%> (ø)
processor/trackingplan.go	95.45% <100.00%> (ø)
processor/transformer/transformer.go	90.79% <100.00%> (ø)
warehouse/api/http.go	78.94% <100.00%> (ø)
warehouse/app.go	89.59% <100.00%> (ø)
warehouse/integrations/testhelper/setup.go	99.68% <100.00%> (ø)
warehouse/internal/model/upload.go	100.00% <ø> (ø)
warehouse/internal/repo/repo.go	70.00% <100.00%> (+14.44%)	⬆️
warehouse/internal/repo/upload.go	91.39% <100.00%> (+0.37%)	⬆️
warehouse/integrations/testhelper/verify.go	89.64% <91.66%> (ø)
... and 9 more

... and 11 files with indirect coverage changes

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