Add Span InMemoryExporter

[tidal]

This PR adds an Span InMemoryExporter for testing and debug purposes

• Adds InMemoryExporter
• Add InMemoryExporter to list of known exporters in ExporterFactory

related: #451

[codecov]

Codecov Report

Merging #775 (641b517) into main (ae6b620) will increase coverage by 0.07%.
The diff coverage is 100.00%.

❗ Current head 641b517 differs from pull request most recent head 0fdbf73. Consider uploading reports for the commit 0fdbf73 to get more accurate results

@@             Coverage Diff              @@
##               main     #775      +/-   ##
============================================
+ Coverage     78.43%   78.51%   +0.07%     
- Complexity     1331     1337       +6     
============================================
  Files           153      154       +1     
  Lines          3278     3290      +12     
============================================
+ Hits           2571     2583      +12     
  Misses          707      707              

Flag	Coverage Δ
7.4	78.51% <100.00%> (+0.07%)	⬆️
8.0	78.58% <100.00%> (+0.07%)	⬆️
8.1	78.58% <100.00%> (+0.07%)	⬆️

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Impacted Files	Coverage Δ
src/SDK/Trace/ExporterFactory.php	100.00% <100.00%> (ø)
src/SDK/Trace/SpanExporter/InMemoryExporter.php	100.00% <100.00%> (ø)

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