Prevent concurrent ::export() calls in span processor

[Nevay]

Export() will never be called concurrently for the same exporter instance.

This PR calls ::export() in a synchronous loop to prevent concurrent calls to ::export().
Note that this might block the Span::end() call that triggered the export longer. The blocking autoflush behavior of BatchSpanProcessor::onEnd() can be disabled, spans will then only be flushed on explicit ::forceFlush() / ::shutdown() calls.

OnEnd() MUST be called synchronously within the Span.End() API, therefore it should not block or throw an exception.

Disallowing concurrent export calls means that we have to reintroduce support for concurrent exports by changing the return type of ::export() to a promise/future-like value - I plan on working on that after this PR is merged.

Alternative to #787, see also #787 (comment).

[codecov]

Codecov Report

Merging #788 (8905837) into main (3ac64f6) will increase coverage by 0.25%.
The diff coverage is 97.65%.

❗ Current head 8905837 differs from pull request most recent head d309f83. Consider uploading reports for the commit d309f83 to get more accurate results

@@             Coverage Diff              @@
##               main     #788      +/-   ##
============================================
+ Coverage     80.40%   80.66%   +0.25%     
- Complexity     1765     1786      +21     
============================================
  Files           222      222              
  Lines          4516     4592      +76     
============================================
+ Hits           3631     3704      +73     
- Misses          885      888       +3     

Flag	Coverage Δ
7.4	80.66% <97.65%> (+0.26%)	⬆️
8.0	80.69% <97.65%> (+0.25%)	⬆️
8.1	80.69% <97.65%> (+0.25%)	⬆️

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Impacted Files	Coverage Δ
...PI/Common/Instrumentation/InstrumentationTrait.php	94.73% <50.00%> (-5.27%)	⬇️
src/SDK/Trace/SpanProcessorFactory.php	92.85% <87.50%> (-7.15%)	⬇️
src/SDK/Trace/SpanProcessor/BatchSpanProcessor.php	100.00% <100.00%> (ø)
...rc/SDK/Trace/SpanProcessor/SimpleSpanProcessor.php	100.00% <100.00%> (ø)

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[Nevay]

Should be changed to default false before beta to be consistent with async implementations that will only export once the event loop gains control. Using default true for now to keep the scope of the PR small / to not break examples etc.