feat: Add observability tool

[dolphin1999]

Description

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This PR introduces a new observability tool that allows fetching system and query level metrics from the Cloud Monitoring API for AlloyDB instances.

The key changes include:

• A new observability tool that can make authenticated requests to the monitoring API.
• Pre-built configurations in observability.yaml for fetching AlloyDB system and query metrics.
• Unit and integration tests for the new observability tool.
• List of metrics is fetched from the public documentation available https://cloud.google.com/alloydb/docs/reference/system-insights-metrics
• Manual testing is also done by locally running the server/tools and integrating with Gemini CLI

Followup Changes

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• Similar observability tools for Postgres, MySQL and SQLServer.
• Documentation around the tools.

PR Checklist

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Thank you for opening a Pull Request! Before submitting your PR, there are a
few things you can do to make sure it goes smoothly:

• Make sure you reviewed
  CONTRIBUTING.md (https://github.com/googleapis/genai-toolbox/blob/main/CONTRIBUTING.md)
• Make sure to open an issue as a
  bug/issue (https://github.com/googleapis/langchain-google-alloydb-pg-python/issues/new/choose)
  before writing your code! That way we can discuss the change, evaluate
  designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• [] Appropriate docs were updated (if necessary)
• Make sure to add ! if this involve a breaking change