Expose complete Cloud Pub/Sub messages through PubsubIO API #20066
Description
The PubsubIO API only exposes a subset of the fields in the underlying PubsubMessage protocol buffer. To accomodate future feature changes as well as for greater compatability with code using the Cloud Pub/Sub apis, a method to read and write these protocol messages should be exposed.
Imported from Jira BEAM-8932. Original Jira may contain additional context.
Reported by: dpcollins-google.