docs(samples): snippet to get `session_id` from WebhookRequest

[hjosiah]

fixes isue #577

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Fixes #577 🦕

[snippet-bot[bot]]

Here is the summary of changes.

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• samples/snippets/webhook_log_session_info.py:15, tag dialogflow_webhook_log_session_id

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[parthea]

@hjosiah As of https://github.com/googleapis/python-dialogflow/pull/630, handwritten samples have migrated to https://github.com/GoogleCloudPlatform/python-docs-samples/tree/main/dialogflow. I'm going to close this PR as obsolete, please could you open a PR in https://github.com/GoogleCloudPlatform/python-docs-samples?