[API/SDK] Provider cleanup

[marcalff]

Fixes #2506

Changes

Please provide a brief description of the changes here.

• Removed methods related to Flush() and Close() from the API class opentelemetry::trace::Tracer, starting with ABI version 2.
  • These methods are not intended for the API, and got exposed as an oversight
  • These methods belong to the SDK instead
• Changed all XXXProviderFactory::Create() methods to return an SDK object instead of an API object.
  • Returning an SDK object is necessary for the application code to perform a proper initialisation, and proper shutdown of the SDK
  • In particular, returning an SDK object is required for the caller to invoke Flush() and Shutdown().
• Removed static_cast<SDK*>(API object) from examples, and use cleaner code, to initialize SDK providers.

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

Attention: Patch coverage is 50.00000% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 87.53%. Comparing base (497eaf4) to head (f201ba1).
Report is 64 commits behind head on main.

❗ Current head f201ba1 differs from pull request most recent head 17ae54d

Please upload reports for the commit 17ae54d to get more accurate results.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2664      +/-   ##
==========================================
+ Coverage   87.12%   87.53%   +0.41%     
==========================================
  Files         200      188      -12     
  Lines        6109     5842     -267     
==========================================
- Hits         5322     5113     -209     
+ Misses        787      729      -58     

Files	Coverage Δ
api/include/opentelemetry/trace/noop.h	93.34% <ø> (ø)
api/include/opentelemetry/trace/tracer.h	100.00% <ø> (ø)
sdk/include/opentelemetry/sdk/trace/tracer.h	100.00% <ø> (ø)
sdk/src/logs/event_logger_provider_factory.cc	100.00% <100.00%> (ø)
sdk/src/trace/tracer_provider_factory.cc	44.83% <42.86%> (+0.39%)	⬆️

... and 55 files with indirect coverage changes

[lalitb]

Good catch :)

[marcalff]

Waiting for the build to finish to make sure DLL links, then this will be ready for review.

[marcalff]

Add help-wanted label:

The windows DLL build fails with undefined symbols, and I can not figure out why (I don't have windows locally to investigate).