trace: Add arguments struct to subscriber::Record (#955)

This branch changes the `Subscriber::record` method to take a new
arguments struct, `span::Record`. The `field::Record` trait was renamed
to `field::Visit` to prevent name conflicts.

In addition, the `ValueSet::is_empty`, `ValueSet::contains`, and
`ValueSet::record` methods were made crate-private, as they are exposed
on the `Attributes` and `Record` types. 

Signed-off-by: Eliza Weisman <[email protected]>

tokio-trace/src/lib.rs

@@ -247,10 +247,10 @@
 //! #[macro_use]
 //! extern crate tokio_trace;
 //! # pub struct FooSubscriber;
-//! # use tokio_trace::{span::{Id, Attributes}, Metadata, field::ValueSet};
+//! # use tokio_trace::{span::{Id, Attributes, Record}, Metadata};
 //! # impl tokio_trace::Subscriber for FooSubscriber {
 //! #   fn new_span(&self, _: &Attributes) -> Id { Id::from_u64(0) }
-//! #   fn record(&self, _: &Id, _: &ValueSet) {}
+//! #   fn record(&self, _: &Id, _: &Record) {}
 //! #   fn event(&self, _: &tokio_trace::Event) {}
 //! #   fn record_follows_from(&self, _: &Id, _: &Id) {}
 //! #   fn enabled(&self, _: &Metadata) -> bool { false }

tokio-trace/src/span.rs

@@ -135,7 +135,7 @@
 //! the data for future use, record it in some manner, or discard it completely.
 //!
 //! [`Subscriber`]: ::Subscriber
-pub use tokio_trace_core::span::{Attributes, Id};
+pub use tokio_trace_core::span::{Attributes, Id, Record};
 
 use std::{
     borrow::Borrow,
@@ -324,7 +324,7 @@ impl<'a> Span<'a> {
             .is_some()
     }
 
-    /// Records that the field described by `field` has the value `value`.
+    /// Visits that the field described by `field` has the value `value`.
     pub fn record<Q: ?Sized, V>(&mut self, field: &Q, value: &V) -> &mut Self
     where
         Q: field::AsField,
@@ -343,7 +343,7 @@ impl<'a> Span<'a> {
         self
     }
 
-    /// Record all the fields in the span
+    /// Visit all the fields in the span
     pub fn record_all(&mut self, values: &field::ValueSet) -> &mut Self {
         if let Some(ref mut inner) = self.inner {
             inner.record(&values);
@@ -479,7 +479,7 @@ impl<'a> Inner<'a> {
 
     fn record(&mut self, values: &field::ValueSet) {
         if values.callsite() == self.meta.callsite() {
-            self.subscriber.record(&self.id, &values)
+            self.subscriber.record(&self.id, &Record::new(values))
         }
     }