Consider tagging module "go"?

I found that go packages, say `github.com/cncf/xds/go/udpa/annotations`, are placed in submodule `github.com/cncf/xds/go`.

However, it seems that module `github.com/cncf/xds/go` is not tagged. According to [Go Modules wiki](https://github.com/golang/go/wiki/Modules#is-it-possible-to-add-a-module-to-a-multi-module-repository), submodule should be tagged like `relative-path-to-root/vX.X.X`.
At now, when trying to import package `github.com/cncf/xds/go/udpa/annotations`, downstream would depends on **pseudo-version** of module `github.com/cncf/xds/go`.
```
github.com/cncf/xds/go v0.0.0-20211001041855-01bcc9b48dfe
```
I think it is not very readable and difficult to upgrade. This is not conductive to version control either.
So, I propose whether it is possible to tag submodule properly to better support go users. For example, `go/v0.0.1`, `go/v1.0.0`etc, so that other project can use tag to import this module in go.mod.