Group otlp spans by resource

[Nevay]

Resolves #712.

I left SpanConverterInterface untouched for now - I'm in favor of removing the interface as span converters of different exporters are not interoperable.

[codecov]

Codecov Report

Merging #740 (26e7f6b) into main (739f212) will decrease coverage by 0.04%.
The diff coverage is 98.96%.

@@             Coverage Diff              @@
##               main     #740      +/-   ##
============================================
- Coverage     82.55%   82.51%   -0.05%     
+ Complexity     1272     1263       -9     
============================================
  Files           146      145       -1     
  Lines          3130     3128       -2     
============================================
- Hits           2584     2581       -3     
- Misses          546      547       +1     

Flag	Coverage Δ
7.4	82.51% <98.94%> (-0.05%)	⬇️
8.0	82.57% <98.96%> (-0.05%)	⬇️
8.1	82.57% <98.96%> (-0.05%)	⬇️

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Impacted Files	Coverage Δ
src/Contrib/Otlp/SpanConverter.php	99.14% <98.96%> (-0.86%)	⬇️

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