attributes: don't record primitive types of the function arguments as fmt::Debug

[Folyd]

Motivation

The default behavior of tracing::instrument attribution will record all of the function arguments as fmt::Debug, which is overwhelmed and unnecessary for those primitive types, such as bool, u8, i8, u16, i16, u32, i32,
u64, i64, usize, and isize. Another concerning reason is that we‘ll lose the type info of those primitive types when record by a Visitor, while those type infos is essential to some people. For example, I need to show my spans in Jaeger UI.

Solution

Make the tracing::instrument records other function arguments as fmt::Debug while not for those primitive types.

However, I'm not good at naming. Maybe the RecordType enum and its variant aren't a good name? I'd love to seek suggestions. Thanks.

[Folyd]

Here is the macro expansion diff of a simple function:

#[tracing::instrument] 
fn hard_work(unit: u64, a: u8) {
}

            ::tracing::Span::new(meta, &{
                #[allow(unused_imports)]
                use ::tracing::field::{debug, display, Value};
                let mut iter = meta.fields().iter();
                meta.fields().value_set(&[
                    (
                        &iter.next().expect("FieldSet corrupted (this is a bug)"),
-                       Some(&tracing::field::debug(&unit) as &Value),
+                       Some(&unit as &Value),
                    ),
                    (
                        &iter.next().expect("FieldSet corrupted (this is a bug)"),
-                      Some(&tracing::field::debug(&a) as &Value),
+                      Some(&a as &Value),
                    ),
                ])
            })

[hawkw]

Thanks for working on this --- overall, this change looks great. I had a few minor suggestions, most of which were documentation-related.

Also, would you mind moving some of the unrelated changes to separate PRs? I'd prefer to keep each commit in the git history as focused as possible. Thank you!

[Folyd]

BTW, does this should be ported to v0.1.x?

[CAD97]

I believe this was a sneakily breaking change on 0.1.X, in that instrumented argument captures of &str changed from matching field filter regexes of "value" to matching value.