Move implementation from SDK header files to SDK cc

[perhapsmaple]

Contributes to #1429

Changes

Please provide a brief description of the changes here.

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

❌ Patch coverage is 87.75510% with 24 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.09%. Comparing base (1b3733f) to head (9692c92).

Files with missing lines	Patch %	Lines
sdk/src/metrics/view/view_registry.cc	82.46%	10 Missing ⚠️
sdk/src/metrics/aggregation/drop_aggregation.cc	0.00%	7 Missing ⚠️
sdk/src/trace/multi_recordable.cc	89.40%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3887      +/-   ##
==========================================
+ Coverage   90.06%   90.09%   +0.03%     
==========================================
  Files         226      229       +3     
  Lines        7223     7253      +30     
==========================================
+ Hits         6505     6534      +29     
- Misses        718      719       +1     

Files with missing lines	Coverage Δ
...lemetry/sdk/metrics/aggregation/drop_aggregation.h	66.67% <ø> (+44.45%)	⬆️
...ude/opentelemetry/sdk/metrics/view/view_registry.h	100.00% <100.00%> (+18.65%)	⬆️
sdk/include/opentelemetry/sdk/trace/span_data.h	100.00% <ø> (ø)
sdk/src/common/empty_attributes.cc	100.00% <100.00%> (ø)
sdk/src/trace/span_data.cc	100.00% <100.00%> (ø)
sdk/src/metrics/aggregation/drop_aggregation.cc	0.00% <0.00%> (ø)
sdk/src/trace/multi_recordable.cc	89.40% <89.40%> (ø)
sdk/src/metrics/view/view_registry.cc	82.46% <82.46%> (ø)

... and 1 file with indirect coverage changes

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[marcalff]

Thanks for the fix.

This has a good size already, so please stabilize this PR to make it ready for review and merge. More files can be changed in a different PR, if so inclined.

To fix:

• IWYU build failures
  • look for "Warning: include-what-you-use reported diagnostics:" in the build logs, in section iwyu_tool
• Resolve merge conflicts
• clang-format

[lalitb]

GetEmptyAttributes() was previously a static free function (header-only, internal linkage). It's now an externally-linked symbol requiring linkage against opentelemetry_common. While this shouldn't be an issue in practice, someone could theoretically be including this header standalone without linking the SDK - for their own utility purposes - and that would now fail with an undefined symbol error. Please add a CHANGELOG entry noting this change.

[lalitb]

Thanks for the PR. LGTM once the CI and merge conflict are resolved.

[ThomsonTan]

Move this and SpanDataLink::GetAttributes to .cc for consistency, since SpanData::GetAttributes has been moved to .cc.