tracing: add index API for Field (#2820)

## Motivation

Expose the index of the field in order to support cases such as sending field information
by index instead of by the string name in Tokio Console. Details:
tokio-rs/console#462 (comment)

## Solution

Adds a new API which exposes the index of a field, which is how it is stored internally
(together with a reference to the `FieldSet` containing the ordered field names.

Signed-off-by: hi-rustin <[email protected]>
Co-authored-by: Hayden Stainsby <[email protected]>

tracing-core/src/field.rs

@@ -681,6 +681,11 @@ impl Field {
     pub fn name(&self) -> &'static str {
         self.fields.names[self.i]
     }
+
+    /// Returns the index of this field in its [`FieldSet`].
+    pub fn index(&self) -> usize {
+        self.i
+    }
 }
 
 impl fmt::Display for Field {
@@ -1053,6 +1058,17 @@ mod test {
         assert!(valueset.is_empty());
     }
 
+    #[test]
+    fn index_of_field_in_fieldset_is_correct() {
+        let fields = TEST_META_1.fields();
+        let foo = fields.field("foo").unwrap();
+        assert_eq!(foo.index(), 0);
+        let bar = fields.field("bar").unwrap();
+        assert_eq!(bar.index(), 1);
+        let baz = fields.field("baz").unwrap();
+        assert_eq!(baz.index(), 2);
+    }
+
     #[test]
     fn empty_value_set_is_empty() {
         let fields = TEST_META_1.fields();