[API] Fix TraceState::Set() appending a duplicate instead of overwriting

[om7057]

Fixes #4583

Changes

TraceState::Set() always allocated room for and appended a new entry, even when the given key already existed in the list. Per the W3C trace-context spec, "only one entry per key is allowed", and vendors must overwrite their entry on reentry, but Set() never checked for an existing entry before appending, so calling it repeatedly with the same key just kept adding duplicates.

Two visible effects, both matching the issue:

• ToHeader() emits the same key more than once, which is not a valid trace-context header.
• Once enough duplicate entries accumulate, kMaxKeyValuePairs is reached and any later, genuinely new key gets silently dropped, even though the list logically has room.

The fix checks whether the key already exists first: an update reuses the existing slot (list size does not grow), and the stale entry for that key is skipped when copying the remaining entries over, so the list only ever holds one entry per key. The already-documented "list full" behavior for a real new key is unchanged.

Added TraceStateTest.TraceStateSetOverwritesExistingKey, which sets the same key many times past kMaxKeyValuePairs and checks both that the value is updated and that a subsequent distinct key can still be added afterward. Confirmed it fails against the pre-fix code (it fills up with 30 duplicate entries and the new key gets dropped) and passes after the fix.

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

[marcalff]

@om7057

Thanks, but this is duplicated effort with #4586.

When working on an issue like #4583, you can check in the issue page, just below the title, if there is a PR already associated with it (hint: saying "Fixes #number" in the PR automatically links the issue).

cc @dbarker

[om7057]

You're right, my mistake for overlooking. I didn't check for a linked PR before starting. Closing this in favor of #4586.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.52%. Comparing base (01dfc71) to head (391012c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4610      +/-   ##
==========================================
+ Coverage   86.51%   86.52%   +0.02%     
==========================================
  Files         525      525              
  Lines       20464    20468       +4     
==========================================
+ Hits        17702    17708       +6     
+ Misses       2762     2760       -2     

Files with missing lines	Coverage Δ
api/include/opentelemetry/trace/trace_state.h	98.93% <100.00%> (+0.05%)	⬆️

... and 1 file with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.