Fix #1867 Orderly shutdown in examples

[marcalff]

Fixes #1867

Changes

Implemented explicit cleanup for:

• The global Tracer provider
• The global Meter provider
• The global Logger provider

in examples.

This is needed to shutdown opentelemetry-cpp in a controlled way.

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

[codecov]

Codecov Report

Merging #1868 (a3f3373) into main (d5f0c48) will decrease coverage by 0.08%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1868      +/-   ##
==========================================
- Coverage   85.80%   85.72%   -0.07%     
==========================================
  Files         171      171              
  Lines        5243     5243              
==========================================
- Hits         4498     4494       -4     
- Misses        745      749       +4     

Impacted Files	Coverage Δ
ext/src/http/client/curl/http_client_curl.cc	80.31% <0.00%> (-1.13%)	⬇️
sdk/src/trace/batch_span_processor.cc	90.70% <0.00%> (-0.77%)	⬇️

[lalitb]

LGTM

[marcalff]

Changed this PR as ready for review, and ready for merge:

• the cleanup has merits on its own, cleaning up the signal provider singleton before shutting down,
• the explicit sdk::init() and sdk::cleanup() calls, discussed and needed for other reasons, will be introduced with another PR, with separate review and discussion.
• the two are orthogonal in my opinion, so this PR can be merged independently.

[ThomsonTan]

Do we still need sdk::init() and sdk::cleanup()? I think yes?