[API] TraceState::Set overwrites an entry with the same key

[dbarker]

Fixes #4583

Changes

• Add a KeyValueProperties::HasKey method to prevent unneeded value copies
• Update TraceState::Set to overwrite an existing entry if the key matches
• Update TraceState::Delete to remove unnecessary kv value copies.
• Update TraceState::FromHeader to discard duplicate keys.
• Add unit tests coverage of the above

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.52%. Comparing base (f5aef4f) to head (e897e52).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4586      +/-   ##
==========================================
+ Coverage   86.51%   86.52%   +0.01%     
==========================================
  Files         525      525              
  Lines       20464    20475      +11     
==========================================
+ Hits        17702    17713      +11     
  Misses       2762     2762              

Files with missing lines	Coverage Δ
api/include/opentelemetry/common/kv_properties.h	98.93% <100.00%> (+0.07%)	⬆️
api/include/opentelemetry/trace/trace_state.h	98.95% <100.00%> (+0.08%)	⬆️

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[dbarker]

This change to Set is an observable behavior change but also a correctness change. Before if a key passed to Set was an existing key for an entry then a new entry would be added with the same key. This violates the spec that keys must be unique.

[dbarker]

This change to delete is not an observable behavior change. The intent is to improve performance by replacing the GetValue call with HasKey to prevent unnecessary value copies.

[marcalff]

LGTM, see a minor optimization comment.