We've it in two places: https://docs.pagure.org/koji/writing_koji_code/#plugins https://docs.pagure.org/koji/writing_a_plugin/
It is almost the same, while the latter contains a few updates. I would replace text in writing koji code with include of the latter file.
writing koji code
include
PR #1794
Metadata Update from @tkopecek: - Custom field Size adjusted to None
Commit b6074963 fixes this issue
Commit 64566ca1 fixes this issue
This issue has been migrated to Fedora Forge: https://forge.fedoraproject.org/koji/koji/issues/1793
Please continue any further discussion there.