Don't go through Display for &str tracing fields

[jplatte]

The extra % was piping the values through tracing::field::display, which makes little sense for &strs, and would lead to quotes being omitted in places where they would be helpful (for example tracing_subscribers fmt::Subscriber).

[tamasfe]

Not specifying the value will just reserve empty fields for them, so it should be path = path in the macro to actually use the values. It's a footgun with tracing's current defaults.

[jplatte]

I thought I used this style successfully before. I'll test it later.

[jplatte]

Dang, you're right. I will send a PR to fix it soon.