[SDK] Valgrind errors on std::atomic variables

[marcalff]

Fixes #2243

Changes

Using only the default constructor to initialize std::atomic objects is incorrect.
This is a particularity of std::atomic itself, that does not follow the same rules as C++ in general.

See: https://en.cppreference.com/w/cpp/atomic/atomic/atomic

In the entire code base, reviewed every use of std::atomic,
and changed initialization to use the direct list initialization syntax.

see: https://en.cppreference.com/w/cpp/language/list_initialization

This form is the most robust and maintainable:

• auditing the source code for correctness is easier
• this form guarantees a proper initialization will be done, in all cases

This fixed the bug reported on the periodic metric reader, tested under valgrind.

This fixed a missing initialization in one of the BatchLogRecordProcessor constructors,
found by code review.

This also fixed the following pattern, which technically, is incorrect:

std::atomic<bool> flag;
flag.store(true);

With the fix, code such as:

std::atomic<bool> flag{false};
flag.store(true);

is correct, because it is now legal to call store() as the object is initialized.

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

[codecov]

Codecov Report

Merging #2244 (65001c2) into main (92a8a54) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2244      +/-   ##
==========================================
- Coverage   87.34%   87.30%   -0.03%     
==========================================
  Files         169      169              
  Lines        4902     4897       -5     
==========================================
- Hits         4281     4275       -6     
- Misses        621      622       +1     

Impacted Files	Coverage Δ
sdk/src/trace/batch_span_processor.cc	92.43% <100.00%> (-0.27%)	⬇️

... and 1 file with indirect coverage changes

[esigo]

LGTM
thanks for the fix :)