[EXPORTER] General cleanup for is_shutdown_ flags in exporters.

[marcalff]

This is a follow up on PR #2553, to simplify bool is_shutdown_ in general.

Changes

Please provide a brief description of the changes here.

• Replace every use of a spin lock + boolean by an std::atomic<bool>
• Replace a spin lock by a mutex, when serializing to a file in the ostream metric exporter
• Implemented the missing call to sout_.flush() in OStreamMetricExporter::ForceFlush()

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

[lalitb]

Thanks for the cleanup.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 87.54%. Comparing base (497eaf4) to head (60ce176).
Report is 60 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2663      +/-   ##
==========================================
+ Coverage   87.12%   87.54%   +0.42%     
==========================================
  Files         200      188      -12     
  Lines        6109     5840     -269     
==========================================
- Hits         5322     5112     -210     
+ Misses        787      728      -59     

Files	Coverage Δ
...lemetry/exporters/memory/in_memory_span_exporter.h	89.48% <100.00%> (-1.43%)	⬇️
...de/opentelemetry/exporters/ostream/span_exporter.h	100.00% <ø> (ø)
exporters/ostream/src/log_record_exporter.cc	97.06% <ø> (-0.08%)	⬇️
exporters/ostream/src/span_exporter.cc	87.88% <ø> (-0.24%)	⬇️
exporters/ostream/src/metric_exporter.cc	90.91% <33.34%> (-0.89%)	⬇️

... and 55 files with indirect coverage changes