auto-regenerate .pb.go files (googleapis#375)

This is an auto-generated regeneration of the .pb.go files by
cloud.google.com/go/internal/gapicgen. Once this PR is submitted, genmgr will
update the corresponding CL at gocloud to depend on the newer version of
go-genproto, and assign reviewers. Whilst this or any regen PR is open in
go-genproto, gapicgen will not create any more regeneration PRs or CLs. If all
regen PRs are closed, gapicgen will create a new set of regeneration PRs and
CLs once per night.

If you have been assigned to review this CL, please:

- Ensure that CI is passing. If it's failing, it requires your manual attention.
- Approve and submit this PR if you believe it's ready to ship. That will prompt
  genmgr to assign reviewers to the gocloud CL.

Corresponding gocloud CL: https://code-review.googlesource.com/c/gocloud/+/56930