sample-event: try synchronous kafka producing

[JoshFerge]

• when creating a sample event, try producing synchronously, since when done through the sample_event endpoint, we may not finish flushing before the request thread is terminated.
• can test this on S4S to see if fixes issue, and quickly revert before it makes it to canary before it can cause problems, but I think it should be fine?

Note: my understanding of kafka / how Django uWSGI lifecycles work is not expert, so my analysis/assumptions here may be off.

[codecov]

Codecov Report

Merging #54748 (7e204dc) into master (43088e2) will increase coverage by 0.00%.
Report is 1 commits behind head on master.
The diff coverage is 0.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #54748   +/-   ##
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  Coverage   79.78%   79.78%           
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  Files        5000     5000           
  Lines      212248   212249    +1     
  Branches    36162    36162           
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+ Hits       169340   169342    +2     
+ Misses      37700    37696    -4     
- Partials     5208     5211    +3     

Files Changed	Coverage Δ
src/sentry/eventstream/kafka/backend.py	68.75% <0.00%> (-0.88%)	⬇️
src/sentry/eventstream/snuba.py	85.00% <ø> (ø)

... and 2 files with indirect coverage changes