Add tests for the new attributes

Bugfix for docs that led me astray for a good half hour :)

The docs on metric labels suggests that they should probably be strings,
and all others I can find are strings, and so these ought to be also.
Otherwise, some of the exporters/processors have to handle things
specifically, and not all of these come out as nice as could be when you
`str()` them.

I've also made sure to use the `StatusCode` name, as that's the
interesting thing.

Finally, there's no need to report specifically that `error=false`, so
I've removed that tag.

Hey, good thing there are tests.

Also, I did the `sorted()` thing in here so future us don't have to
think about the ordering of these when writing new tests :)

This code passed before, so something clearly changed.