[SDK] Fix not reporting failures from MultiSpanProcessor/MultiLogRecordProcessor flush/shutdown

[denizariyan]

Fixes #4471

Changes

MultiSpanProcessor::ForceFlush, MultiSpanProcessor::InternalShutdown and MultiLogRecordProcessor::InternalShutdown walk their children accumulating a bool result that is initialised to true and then combined with |=, so no child can ever move it off true.

• Replaced the three |= accumulations with if (!child) { result = false; }. If statement results in same instructions (i.e., no added branching) and is more readable and harder to get wrong.
  • TracerContext/LoggerContext wrap the processor vector in a Multi* unconditionally, so this was returning a meaningless true even for users who configured a single processor.
• MultiSpanProcessor had no test file anywhere in the tree and MultiLogRecordProcessor's only coverage was three Enabled-related tests living inside simple_log_record_processor_test.cc. I added multi_span_processor_test.cc and multi_log_record_processor_test.cc, and moved those three existing tests into the latter so each Multi* component is tested from a file named after it. The moved tests are unchanged apart from the suite name and dropping the now-redundant MultiLogRecordProcessor prefix from their names.

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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.13%. Comparing base (cd252b4) to head (8e15293).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4472      +/-   ##
==========================================
+ Coverage   83.09%   83.13%   +0.04%     
==========================================
  Files         519      519              
  Lines       20253    20256       +3     
==========================================
+ Hits        16827    16837      +10     
+ Misses       3426     3419       -7     

Files with missing lines	Coverage Δ
...ude/opentelemetry/sdk/trace/multi_span_processor.h	100.00% <100.00%> (+6.58%)	⬆️
sdk/src/logs/multi_log_record_processor.cc	96.48% <100.00%> (+0.05%)	⬆️

... and 1 file with indirect coverage changes

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[dbarker]

Thanks for the fix! Appreciate the test cleanup also.