[CODE HEALTH] Fix clang-tidy warnings, part 1.

[marcalff]

Fixes # (issue)

Changes

Please provide a brief description of the changes here.

• Fixes cppcoreguidelines-explicit-virtual-functions warnings.

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 75.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.96%. Comparing base (46e7619) to head (4cc8ddf).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
api/include/opentelemetry/nostd/variant.h	0.00%	1 Missing ⚠️
...lemetry/exporters/memory/in_memory_span_exporter.h	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3841      +/-   ##
==========================================
+ Coverage   89.95%   89.96%   +0.02%     
==========================================
  Files         225      225              
  Lines        7170     7169       -1     
==========================================
  Hits         6449     6449              
+ Misses        721      720       -1     

Files with missing lines	Coverage Δ
...telemetry/sdk/metrics/state/multi_metric_storage.h	100.00% <100.00%> (ø)
...ude/opentelemetry/sdk/trace/multi_span_processor.h	100.00% <100.00%> (ø)
api/include/opentelemetry/nostd/variant.h	66.67% <0.00%> (ø)
...lemetry/exporters/memory/in_memory_span_exporter.h	85.00% <0.00%> (+4.05%)	⬆️

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[lalitb]

thanks,