Fix: GCP flush before error thrown.

[lucas-zimerman]

This adds a new flush method called FlushBeforeRequestCompleted, instead of flushing after the request is sent like in FlushOnCompletedRequest, this flag sends the envelopes before sending the response to the user.

It's a required flow for some serverless/Faas platforms that will kill the process after flushing the HttpResponse to the receiver.

I made it internal for now and ideally in the future we could have a simple enum that will set the flush type (none, on completed, before completed).

Close #1213

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.76%. Comparing base (6f14500) to head (e32e05e).
⚠️ Report is 2146 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1249      +/-   ##
==========================================
+ Coverage   80.61%   80.76%   +0.14%     
==========================================
  Files         212      212              
  Lines        6913     6920       +7     
  Branches     1591     1596       +5     
==========================================
+ Hits         5573     5589      +16     
+ Misses        809      803       -6     
+ Partials      531      528       -3     

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