this was introduced in #9159 and help solve case like this where we need to prevent pandoc from parsing as Markdown the metadata fields passed in the YAML file used with `metadata-file:`

For context about `--metadata-file` see https://pandoc.org/MANUAL.html#option--metadata-file
> Read metadata from the supplied YAML (or JSON) file. This option can be used with every input format, but string scalars in the metadata file will always be parsed as Markdown.

from #9206

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