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 :white_check_mark:

Project coverage is 70.83%. Comparing base (a84c048) to head (c5ff850). Report is 863 commits behind head on main.

Additional details and impacted files

```diff @@ 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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