Add sdk language to span metrics

[grcevski]

The program language is low cardinality and it's quite useful to be part of span metrics, avoiding expensive joins with target_info.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 55.12%. Comparing base (7e3c547) to head (57b2da3).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/appolly/app/request/span_getters.go	0.00%	3 Missing ⚠️
pkg/export/prom/prom.go	0.00%	2 Missing ⚠️
pkg/export/otel/metrics.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #908      +/-   ##
==========================================
- Coverage   55.14%   55.12%   -0.02%     
==========================================
  Files         253      253              
  Lines       21719    21725       +6     
==========================================
- Hits        11976    11975       -1     
- Misses       8920     8926       +6     
- Partials      823      824       +1     

Flag	Coverage Δ
integration-test	23.26% <66.66%> (-0.03%)	⬇️
integration-test-arm	0.00% <0.00%> (ø)
integration-test-vm-${ARCH}-${KERNEL_VERSION}	0.00% <0.00%> (?)
k8s-integration-test	2.73% <0.00%> (-0.01%)	⬇️
oats-test	0.00% <0.00%> (ø)
unittests	46.23% <66.66%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[marctc]

LGTM