Add gemini token counts

[umaannamalai]

This PR adds response.usage.completion_tokens, response.usage.prompt_tokens, response.usage.total_tokens attributes back into LLM events.

[github-actions]

🦙 MegaLinter status: ❌ ERROR

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ ACTION	actionlint	6		0	0	0.41s
✅ MARKDOWN	markdownlint	7	0	0	0	0.98s
✅ MARKDOWN	markdown-link-check	7		0	0	14.16s
❌ PYTHON	ruff	908	1	1	0	0.6s
✅ PYTHON	ruff-format	908	1	0	0	0.33s
✅ YAML	prettier	13	0	0	0	1.23s
✅ YAML	v8r	13		0	0	5.8s
✅ YAML	yamllint	13		0	0	0.55s

See detailed report in MegaLinter reports

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 69.56522% with 14 lines in your changes missing coverage. Please review.

Please upload report for BASE (feature-gemini-instrumentation@bac3ede). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
newrelic/hooks/mlmodel_gemini.py	69.56%	9 Missing and 5 partials ⚠️

Additional details and impacted files

@@                        Coverage Diff                        @@
##             feature-gemini-instrumentation    #1377   +/-   ##
=================================================================
  Coverage                                  ?   81.30%           
=================================================================
  Files                                     ?      205           
  Lines                                     ?    23230           
  Branches                                  ?     3671           
=================================================================
  Hits                                      ?    18886           
  Misses                                    ?     3123           
  Partials                                  ?     1221           

☔ 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.