docs(samples): add set agent code sample

[galdawave]

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:

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Fixes #<issue_number_goes_here> 🦕

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• samples/snippets/set_agent.py:15, tag dialogflow_set_agent_sample

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[tmatsuo]

Suggestion: The test code might confuse the readers. Maybe you can add a comment justifying the compromising test strategy.