core: implement field::ValidLen for slices

There used to be a length restriction on the arrays usable to construct
`ValueSet`s, but this was due to Rust compiler limitations that no
longer exist. After that restriction was removed, the `ValidLen` trait
was kept around but implemented for statically-sized arrays of all
lengths.

While in theory this allows `ValueSet`s to be constructed from arrays of
arbitrary length, the size of the arrays must still be known at compile
time. Now that there is no longer a restriction on the length of arrays,
slices could in theory be used as well (since they meet the supertrait
requirement of `ValidLen`), but no implementation of the trait exists.

Adding an implementation of `ValidLen` for slices is straightforward.

Refs: tokio-rs#2508

tracing-core/src/field.rs

@@ -1081,6 +1081,7 @@ mod private {
     pub trait ValidLen<'a>: Borrow<[(&'a Field, Option<&'a (dyn Value + 'a)>)]> {}
 
     impl<'a, const N: usize> ValidLen<'a> for [(&'a Field, Option<&'a (dyn Value + 'a)>); N] {}
+    impl<'a> ValidLen<'a> for &[(&'a Field, Option<&'a (dyn Value + 'a)>)] {}
 }
 
 #[cfg(test)]