Add Gemini instrumentation

[umaannamalai]

This PR adds instrumentation for text generations and embeddings generated with Google Gemini via the python-genai package.

[github-actions]

🦙 MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ ACTION	actionlint	6		0	0	0.47s
✅ MARKDOWN	markdownlint	7	0	0	0	1.01s
✅ MARKDOWN	markdown-link-check	7		0	0	15.89s
✅ PYTHON	ruff	908	0	0	0	0.58s
✅ PYTHON	ruff-format	908	0	0	0	0.3s
✅ YAML	prettier	13	0	0	0	1.17s
✅ YAML	v8r	13		0	0	6.23s
✅ YAML	yamllint	13		0	0	0.57s

See detailed report in MegaLinter reports

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 80.53435% with 51 lines in your changes missing coverage. Please review.

Project coverage is 81.57%. Comparing base (c346346) to head (b2d8ac9).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
newrelic/hooks/mlmodel_gemini.py	80.45%	39 Missing and 12 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1379      +/-   ##
==========================================
+ Coverage   81.32%   81.57%   +0.25%     
==========================================
  Files         204      205       +1     
  Lines       22952    23214     +262     
  Branches     3639     3670      +31     
==========================================
+ Hits        18665    18937     +272     
+ Misses       3078     3053      -25     
- Partials     1209     1224      +15     

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