`draft` list in `metadata-files` is ignored

Discussed in https://github.com/quarto-dev/quarto-cli/discussions/10568

^{Originally posted by andrewpbray August 20, 2024}

Description

Hello! I'm running into a puzzling issue when trying to create a metadata extension. I have an extension that I'd like to add metadata-files to the _quarto.yml to set a draft list.

contributes:
  metadata:
    project:
      output-dir: _mysite
    metadata-files:
      - draft-list.yml

When I render this website, the draft list does not appear to have taken effect. It will work, however, if I adjust _quarto.yml to read:

project:
  type: website

website:
  sidebar: 
    contents: 
      - index.qmd
      - about.qmd

metadata-files:
  - draft-list.yml

Which suggests the issue is in merging the metadata-files key in from the extension (the output-dir does take effect).

Minimal example is here: https://github.com/andrewpbray/reprex-metadata-merge

The docs on metadata extensions state "Currently, metadata extensions only merge project-level metadata. This limitation will be lifted in the future." I had assumed "project-level metadata" is anything that appears in _quarto.yml but perhaps its only the keys that are accepted under the project key. I've tried shifting metadata-files so that it's not under the metadata key, but still no luck.

The real use-case here is the scheduled-docs extension. It has a pre and post render script (that run just fine) that generate a draft-list.yml that I'd like to automatically get merge into _quarto.yml without the user needing to manually modify their _quarto.yml.