[hidingmyname]

Fixes #4749

Changes

A span link whose span context is invalid and which has no attributes and no
trace state carries no information. The SDK recorded such links anyway, so an
empty link could take a slot under link_count_limit and push out a real one
(with link_count_limit = 1 and links {empty, valid}, the empty link was
kept and the valid one dropped).

Span now skips these links in all three places links are recorded: links
passed at span creation, and AddLink / AddLinks (ABI v2). An invalid
context that still has attributes or a non-empty trace state is kept, as
before.

This is the same rule used by the Go (recordingSpan.AddLink), Java
(SdkSpan.addLink) and Python (_is_valid_link) SDKs.

Tests: new Tracer.SpanSkipsEmptyLinks in sdk/test/trace/tracer_test.cc:

• link_count_limit = 1, links {empty, valid}: the valid link is recorded
  and 0 links are dropped.
• An invalid context with a trace state, or with attributes, is still recorded.
• ABI v2: AddLink / AddLinks with an empty link record nothing, and a
  following valid link is kept with 0 dropped.

Without the change in span.cc the new test fails (the empty link is
recorded and the valid one dropped), under both ABI v1 and ABI v2 (including
the AddLink / AddLinks case); with it, it passes.

Results (CMake, Debug, GCC 15.3, clang-format 18):

• ABI v1: all trace SDK tests in sdk/test/trace pass (174/174 via ctest;
  tracer_test 45/45).
• ABI v2 (WITH_ABI_VERSION_2=ON): tracer_test 48/48 pass, including
  SpanSkipsEmptyLinks with the AddLink / AddLinks cases and the existing
  SpanAddLinkAbiv2, SpanAddLinksAbiv2, SpanMixLinksAbiv2.

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

[mateenali66]

LGTM, thanks for the fix.

Under ABI v2, SpanSkipsEmptyLinks fails with the filter removed from any of the three call sites, or with either clause of IsEmptyLink dropped.

The rule matches the spec text on links with an empty TraceId or SpanId.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.62%. Comparing base (d05c6a8) to head (d94021e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4772      +/-   ##
==========================================
+ Coverage   86.61%   86.62%   +0.02%     
==========================================
  Files         530      530              
  Lines       21169    21176       +7     
==========================================
+ Hits        18333    18342       +9     
+ Misses       2836     2834       -2     

Files with missing lines	Coverage Δ
sdk/src/trace/span.cc	95.20% <100.00%> (+0.29%)	⬆️

... and 1 file with indirect coverage changes

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