Prevent leaking TracerProvider in register_shutdown_function()

[Nevay]

Fix memory leak caused by keeping strong references to every created TracerProvider in callback provided to register_shutdown_function().

[codecov]

Codecov Report

Merging #716 (6d66e0e) into main (a22f9f1) will decrease coverage by 0.37%.
The diff coverage is 12.50%.

@@             Coverage Diff              @@
##               main     #716      +/-   ##
============================================
- Coverage     82.78%   82.41%   -0.38%     
- Complexity     1241     1247       +6     
============================================
  Files           139      139              
  Lines          3033     3048      +15     
============================================
+ Hits           2511     2512       +1     
- Misses          522      536      +14     

Flag	Coverage Δ
7.4	82.40% <12.50%> (-0.38%)	⬇️
8.0	82.46% <12.50%> (-0.38%)	⬇️
8.1	82.46% <12.50%> (-0.38%)	⬇️

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Impacted Files	Coverage Δ
src/SDK/Trace/TracerProvider.php	72.54% <12.50%> (-27.46%)	⬇️

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