Prevent duplicate DT headers for grpc

[kanderson250]

Resolves #1768

This PR addresses a bug with grpc apps using distributed tracing. Previously our grpc instrumentation allowed duplicate DT headers, which caused DT payloads to be rejected by downstream services. This is now prohibited.

Changes:

• Prevent adding duplicate headers in all grpc instrumentation modules
• Allow parsing of duplicate (same value) parent headers in W3CTraceParentParser. Also add a log message when this happens to help diagnose the issue should it recur.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.83%. Comparing base (a84c048) to head (c5ff850).
Report is 36 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #1783   +/-   ##
=========================================
  Coverage     70.82%   70.83%           
- Complexity     9934     9946   +12     
=========================================
  Files           825      826    +1     
  Lines         39826    39875   +49     
  Branches       6037     6043    +6     
=========================================
+ Hits          28207    28245   +38     
- Misses         8891     8903   +12     
+ Partials       2728     2727    -1     

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