[BEAM-7965] add retracting mode to model proto

[amaliujia]

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Post-Commit Tests Status (on master branch)

Lang	SDK	Apex	Dataflow	Flink	Gearpump	Samza	Spark
Go		---	---		---	---
Java
Python		---			---	---
XLang	---	---	---		---	---	---

Pre-Commit Tests Status (on master branch)

---	Java	Python	Go	Website
Non-portable
Portable	---		---	---

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[amaliujia]

R: @kennknowles @lukecwik

I will hold this PR unitl receive more feedback [1]. This PR is used to see what's the first step to kick off retraction effort. Also it's interesting to know what's the process of adding something to model proto.

[1] : https://lists.apache.org/thread.html/689dd70669660fb60944557e039c18b24148d8a2caf068bf5f07b1dc@%3Cdev.beam.apache.org%3E

[lukecwik]

You'll need to regenerate the go protos following https://github.com/apache/beam/blob/master/sdks/go/pkg/beam/model/PROTOBUF.md

[amaliujia]

Thanks Luke for your pointer!

also R: @lostluck as this change affects Go SDK

[amaliujia]

I think the failed go check is due to new generated proto for Go used 3.x protobuf while current Go SDK uses 2.x protobuf.

I will wait Bobert's feedback on what's the best way to go (upgrade or stay with 2.x).

[lukecwik]

I'm pretty sure that Go is using proto 3 and not proto 2

[amaliujia]

Thanks Luke. I also realized that proto files in model are proto3 already.

Undo re-generated protos for Go.

Thanks Daniel for creating JIRA to track re-generate Go protos in correct version: https://jira.apache.org/jira/browse/BEAM-7970.

I think we can separate this PR's change with re-generation of protos for Go, given the created JIRA to track proto re-generation effort. I will also wait for @lostluck to see if he is also ok on this separation.