Ruby: Convert Descriptor to DescriptorProto

[willbryant]

What language does this apply to?
Ruby, proto2 in particular.

Describe the problem you are trying to solve.

Exactly the same as #9425 for C#, we need to send a DescriptorProto to use the BigQuery Storage write streaming API. The Ruby bindings allow us to generate the message class and get it as a Descriptor, but there doesn't seem to be an API to convert this to a DescriptorProto as required for the proto2 writer_schema argument.

Describe the solution you'd like

Just like #9426 for C#.

Describe alternatives you've considered

Manually constructing a Descriptor that is identical to the message class's fields.

Additional context
Add any other context or screenshots about the feature request here.

[haberman]

It would be relatively trivial to add to_proto methods on the descriptor classes. At the upb level we have an API that is perfect for this: https://github.com/protocolbuffers/upb/blob/main/upb/util/def_to_proto.h

[dazuma]

This has come up a few times for Ruby. There's an ugly workaround described here: googleapis/google-cloud-ruby#19093 but we should provide a real solution.

[github-actions]

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This issue is labeled inactive because the last activity was over 90 days ago.

[willbryant]

Still an issue, bot :)

[haberman]

This would be fairly straightforward to contribute, but I'm not able to prioritize it myself.

[github-actions]

We triage inactive PRs and issues in order to make it easier to find active work. If this issue should remain active or becomes active again, please add a comment.

This issue is labeled inactive because the last activity was over 90 days ago.

[willbryant]

Still an issue.