Feature Highlight: Transaction Metadata

SpiceDB v1.38 introduces Transaction Metadata, a new API functionality that allows users to include arbitrary metadata when writing relationships. This feature is designed as a building block for solving various challenges faced by users and customers. It enables associating more information with changes to the underlying data powering authorization decisions, unlocking numerous potential use cases. The size of metadata is limited to 65k of total storage across all datastores to maintain consistency and compatibility when migrating relationship data. This feature was initially developed in response to a customer's need for observability in asynchronous workflows involving SpiceDB and Kafka streams.