Add ability to configure extra span metrics resource attributes

[grcevski]

This PR allows for users to configure extra resource attributes from trace spans to be added to the span metrics, allowing for more flexible metrics usage. For example, a custom resource attribute service.version can now be added to the span metrics.

[codecov]

Codecov Report

❌ Patch coverage is 9.67742% with 28 lines in your changes missing coverage. Please review.
✅ Project coverage is 55.07%. Comparing base (b3fa99f) to head (2d57c8d).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/export/prom/prom.go	8.33%	22 Missing ⚠️
pkg/export/otel/metrics.go	14.28%	5 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #909      +/-   ##
==========================================
- Coverage   55.13%   55.07%   -0.06%     
==========================================
  Files         253      253              
  Lines       21732    21756      +24     
==========================================
+ Hits        11982    11983       +1     
- Misses       8926     8947      +21     
- Partials      824      826       +2     

Flag	Coverage Δ
integration-test	23.22% <3.22%> (-0.08%)	⬇️
integration-test-arm	0.00% <0.00%> (ø)
integration-test-vm-${ARCH}-${KERNEL_VERSION}	0.00% <0.00%> (ø)
k8s-integration-test	2.74% <0.00%> (+<0.01%)	⬆️
oats-test	0.00% <0.00%> (ø)
unittests	46.21% <9.67%> (-0.04%)	⬇️

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