chore(llmobs): add language tag to span events [backport 2.9]

[github-actions]

Backport b488c99 from #10681 to 2.9.

Adds a language:python tag to all LLMObs span events, to be used for internal analysis. No changelog needed since this isn't a user-facing functional change or fix (it will just show as an extra tag in the UI).

MLOB-1543

Checklist

• PR author has checked that all the criteria below are met
• The PR description includes an overview of the change
• The PR description articulates the motivation for the change
• The change includes tests OR the PR description describes a testing strategy
• The PR description notes risks associated with the change, if any
• Newly-added code is easy to change
• The change follows the library release note guidelines
• The change includes or references documentation updates if necessary
• Backport labels are set (if applicable)

Reviewer Checklist

• Reviewer has checked that all the criteria below are met
• Title is accurate
• All changes are related to the pull request's stated goal
• Avoids breaking API changes
• Testing strategy adequately addresses listed risks
• Newly-added code is easy to change
• Release note makes sense to a user of the library
• If necessary, author has acknowledged and discussed the performance implications of this PR as reported in the benchmarks PR comment
• Backport labels are set in a manner that is consistent with the release branch maintenance policy

[datadog-dd-trace-py-rkomorn]

Datadog Report

Branch report: backport-10681-to-2.9
Commit report: edd6013
Test service: dd-trace-py

✅ 0 Failed, 879 Passed, 39150 Skipped, 35m 33.72s Total duration (2h 2m 29.84s time saved)
⌛ 3 Performance Regressions

⌛ Performance Regressions vs Default Branch (3)

• test_tracer_reconfigured_with_active_span_does_not_crash - test_processors.py 15.09s (+13.44s, +816%)
• test_extract_128bit_trace_ids_tracecontext - test_propagation.py 3.48s (+2.85s, +449%) - Details
• test_inject_128bit_trace_id_b3_single_header - test_propagation.py 3.32s (+2.69s, +427%) - Details