Support datetime and UUID attributes

Contents (#7)

IMPORTANT: There are 2 breaking changes.

Enhancements

• 💥 BREAKING CHANGE: Store event UUID in its own column
• 💥 BREAKING CHANGE: Replace Pub/Sub publish time with backend-agnostic datetime attribute

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Upgrade instructions

Add to your bigquery table:

• A datetime column of the TIMESTAMP type
• A uuid column of the STRING type