Introducing AddLink method in TelemetrySpan

[arenavalerio]

Fixes #6042
Adding an implementation of AddLink method in TelemetrySpan class

Changes

• Adding AddLink method to TelemetrySpan class
• Adding relevant unit tests
• Adding relevant lines for public API changes

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: arenavalerio / name: Valerio Arena (4884e39, 7954194, 8cdd9ad, 042b603, 47cc879, 0b5531b, c49817b, bc08449, 87c48b1, 30f43c4)
• ✅ login: alanwest / name: Alan West (3fedb61)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.78%. Comparing base (1544659) to head (3fedb61).
Report is 1 commits behind head on main.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6305      +/-   ##
==========================================
+ Coverage   86.75%   86.78%   +0.03%     
==========================================
  Files         258      258              
  Lines       11872    11879       +7     
==========================================
+ Hits        10299    10309      +10     
+ Misses       1573     1570       -3     

Flag	Coverage Δ
unittests-Project-Experimental	86.57% <100.00%> (+0.16%)	⬆️
unittests-Project-Stable	86.39% <100.00%> (-0.20%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/OpenTelemetry.Api/Trace/TelemetrySpan.cs	89.47% <100.00%> (+1.06%)	⬆️

... and 2 files with indirect coverage changes

[alanwest]

LGTM. Thanks @arenavalerio!