core: expose an accessor for the FieldSet on Attributes (#1331)

## Motivation

Exposing an accessor for the `FieldSet` on `Attributes` can motivate the
subscriber to achieve some performance improvement, such as
`OpenTelemetrySubscriber` #1327.

## Alternative

Make `ValueSet::field_set()` be `pub` instead of `pub(crate)`.

Co-authored-by: Eliza Weisman <eliza@buoyant.io>

tracing-core/src/span.rs

@@ -1,4 +1,5 @@
 //! Spans represent periods of time in the execution of a program.
+use crate::field::FieldSet;
 use crate::parent::Parent;
 use crate::stdlib::num::NonZeroU64;
 use crate::{field, Metadata};
@@ -196,6 +197,21 @@ impl<'a> Attributes<'a> {
     pub fn is_empty(&self) -> bool {
         self.values.is_empty()
     }
+
+    /// Returns the set of all [fields] defined by this span's [`Metadata`].
+    ///
+    /// Note that the [`FieldSet`] returned by this method includes *all* the
+    /// fields declared by this span, not just those with values that are recorded
+    /// as part of this set of `Attributes`. Other fields with values not present in
+    /// this `Attributes`' value set may [record] values later.
+    ///
+    /// [fields]: crate::field
+    /// [record]: Attributes::record()
+    /// [`Metadata`]: crate::metadata::Metadata
+    /// [`FieldSet`]: crate::field::FieldSet
+    pub fn fields(&self) -> &FieldSet {
+        self.values.field_set()
+    }
 }
 
 // ===== impl Record =====