Add Gemini text generations instrumentation

[umaannamalai]

This PR adds instrumentation and tests for Gemini text generations

[github-actions]

🦙 MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ ACTION	actionlint	4		0	0	0.47s
✅ MARKDOWN	markdownlint	7	0	0	0	0.94s
✅ MARKDOWN	markdown-link-check	7		0	0	17.06s
✅ PYTHON	ruff	900	0	0	0	0.57s
✅ PYTHON	ruff-format	900	0	0	0	0.21s
✅ YAML	prettier	10	0	0	0	0.95s
✅ YAML	v8r	10		0	0	5.25s
✅ YAML	yamllint	10		0	0	0.55s

See detailed report in MegaLinter reports

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 76.72956% with 37 lines in your changes missing coverage. Please review.

Please upload report for BASE (feature-gemini-instrumentation@4f9d3e6). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
newrelic/hooks/mlmodel_gemini.py	76.72%	28 Missing and 9 partials ⚠️

Additional details and impacted files

@@                        Coverage Diff                        @@
##             feature-gemini-instrumentation    #1349   +/-   ##
=================================================================
  Coverage                                  ?   81.45%           
=================================================================
  Files                                     ?      205           
  Lines                                     ?    23088           
  Branches                                  ?     3650           
=================================================================
  Hits                                      ?    18807           
  Misses                                    ?     3055           
  Partials                                  ?     1226           

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[hmstepanek]

Posting a partial review-I'll review the rest tomorrow.

[hmstepanek]

I was trying to check the code coverage report to make sure we covered all the cases but it seems it's not available. Maybe it will be after you push some changes or maybe just take a look at it locally and double check after.