fix: Fix duplicate key error when using Kafka with DT enabled

[chynesNR]

There were actually two bugs when using Kafka with Distributed Tracing:

• If headers were not present, we weren't creating them (due to a scoping error)
• We were not checking to see if the keys already existed before adding them. This did not cause an immediate problem because the headers are kept in a list rather than a dictionary, but can cause a validation failure later.

The first bug kept us from finding the second problem in our integration tests. Both have been addressed.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.46%. Comparing base (d9816fb) to head (cb76d94).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2433      +/-   ##
==========================================
- Coverage   81.44%   80.46%   -0.98%     
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  Files         406      450      +44     
  Lines       25063    27942    +2879     
  Branches     3023     3023              
==========================================
+ Hits        20412    22483    +2071     
- Misses       3877     4685     +808     
  Partials      774      774              

Flag	Coverage Δ
Agent	81.44% <ø> (ø)
Profiler	71.93% <ø> (?)

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see 46 files with indirect coverage changes

[tippmar-nr]

Good fix!