Fix header only api singletons (#1520)

[marcalff]

This fix is for gcc and clang only.

Fixes #1520

Changes

Add compiler annotations to make singleton methods visible.
This is needed by the linker to ensure uniqueness of singletons.

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #1604 (02e78bb) into main (cfaf8a1) will increase coverage by 0.09%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1604      +/-   ##
==========================================
+ Coverage   85.22%   85.30%   +0.09%     
==========================================
  Files         156      156              
  Lines        4978     4978              
==========================================
+ Hits         4242     4246       +4     
+ Misses        736      732       -4     

Impacted Files	Coverage Δ
api/include/opentelemetry/baggage/baggage.h	97.33% <100.00%> (ø)
...ntelemetry/context/propagation/global_propagator.h	100.00% <100.00%> (ø)
...pi/include/opentelemetry/context/runtime_context.h	97.60% <100.00%> (ø)
api/include/opentelemetry/metrics/provider.h	100.00% <100.00%> (ø)
api/include/opentelemetry/trace/provider.h	100.00% <100.00%> (ø)
api/include/opentelemetry/trace/trace_state.h	97.60% <100.00%> (ø)
sdk/src/trace/batch_span_processor.cc	91.41% <0.00%> (+1.57%)	⬆️
api/include/opentelemetry/trace/noop.h	93.11% <0.00%> (+6.90%)	⬆️

[owent]

Should we disable component_c .. component_f and keep component_a and component_b on Windows?

[lalitb]

LGTM, can we merged once conflict is resolved.

[marcalff]

LGTM, can we merged once conflict is resolved.

@lalitb

Thanks.
Merge conflicts now resolved.