pubsub/v1.2.0
- Add support for upcoming dead letter topics feature
- Expose Subscription.ReceiveSettings.MaxExtensionPeriod setting
- Standardize default settings with other client libraries
- Increase publish delay threshold from 1ms to 10ms
- Increase subscription MaxExtension from 10m to 60m
- Always send keepalive/heartbeat ping on StreamingPull streams to minimize
stream reopen requests