Adding github labels/reviewers to a github_pr_destination

[Nipunn Koorapati]

Hi,

May I have some assistance. I'd like to be able to use a `github_pr_destination` to move commits over, with reviewers and github label automatically attached. Is there a good way to do this (or solve my problem in some other way?).

I see https://github.com/google/copybara/blob/master/docs/reference.md#github_api_objadd_label - but cannot quite figure out how to get the PR number.

Thanks

--Nipunn