[hakyll] metadata on Items created with makeItem

[m***@emptypath.com]

How do I specify metadata for an Item that I create from scratch with
`makeItem`?

For example, in this testcase site:
https://github.com/kadoban/hakyll_question

How do I make the output "sitemap.xml" file have "prio set" on the line
that corresponds with "blog.html"?

I can't find any way in the create "blog.html" rule, or elsewhere to do
that without messing up other things.

I assume there's some way to do it, because pandocCompiler must be doing
something along these lines when I pass a .markdown file through it and it
reads the metadata from the top of the markdown file in that block
deliminated by ----s, but I don't see at all in the code for that where
it's happening, or how.

Any help would be much appreciated, thanks!