Update metadata examples for interceptors

[ViridianForge]

          I just ran into this using the `metadata` keyword. Looks like the docs still references this mechanism even though it doesn't seem to work. Can we get the updated example of how to use the interceptors for authorization like this?

Originally posted by @imrimt in #45 (comment)

[parmsib]

I just ran into this. Spent some time wondering why the metadata auth method seen in the documentation wasn't working until I saw the issues discussing this.

[ViridianForge]

Apologies for letting this linger for so long.
@parmsib - did you get auth working for your use case? If so, I'd appreciate a quick pseudocode or PR to update the example.

[ViridianForge]

I think I figured out what ya'll were talking about. Thanks for calling it out.

I'd appreciate a set of eyes on this proposed example before I merge it, just to make sure it makes sense. Might be a few days before I have time to test it outside of the sandbox of the tests.

[parmsib]

No worries, thanks for the useful lib!

Added a few comments on your commit. Mostly nitpicks, so makes sense in general to me.

[ViridianForge]

Thanks for the feedback @parmsib - the updated examples have been rolled in!