Update print statement to use model_dump_json

[Rusbel Bermudez (rb-one)]

Overview

Update documentation: 'The json method is deprecated; the documentation encourage you to use model_dump_json, the current example is broken

Checklist

• I have read the contributing guidelines
• I have tested my changes locally using docs dev
• All code examples have been tested and work correctly
• I have used root relative paths for internal links
• I have updated navigation in src/docs.json if needed

[Copilot]

Pull request overview

This pull request updates a Python code example to use the current Pydantic API method model_dump_json() instead of the deprecated .json() method, fixing a broken code example in the documentation.

Changes:

• Updated the print statement in the experiment metrics example to use model_dump_json() instead of the deprecated .json() method