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 87.81726% with 24 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.06%. Comparing base (78c8f2b) to head (9cde07e).
⚠️ Report is 9 commits behind head on main.

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   89.99%   90.06%   +0.08%     
==========================================
  Files         225      229       +4     
  Lines        7170     7231      +61     
==========================================
+ Hits         6452     6512      +60     
- 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%> (+17.25%)	⬆️
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 8 files with indirect coverage changes

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