Run exports with the Context the SpanProcessor was created in

[Nevay]

Resolves #878.

(Matches the behavior of async implementations that create the export fiber in the constructor, example).

[brettmc]

I tested this out, it works for me - exporter requests are no longer instrumented. I can see that the instrumentation handler pre function now gets a no-op tracer when handling exporting requests.

[codecov]

Codecov Report

Merging #880 (98c8bfa) into main (0ce2fda) will increase coverage by 0.02%.
The diff coverage is 100.00%.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #880      +/-   ##
============================================
+ Coverage     81.21%   81.23%   +0.02%     
  Complexity     2008     2008              
============================================
  Files           264      264              
  Lines          5212     5218       +6     
============================================
+ Hits           4233     4239       +6     
  Misses          979      979              

Flag	Coverage Δ
7.4	80.72% <100.00%> (+0.02%)	⬆️
8.0	81.22% <100.00%> (+0.02%)	⬆️
8.1	81.36% <100.00%> (+0.02%)	⬆️

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Impacted Files	Coverage Δ
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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