tracing: inline methods to make macro-generated code smaller (#2555)

## Motivation

Make `tracing::event!` codegen smaller

## Solution

Add `inline` to several functions called by `tracing::event!`.

Simple example: https://github.com/ldm0/tracing_test

After inlining, executable size drops from 746kb to 697kb
(`cargo build --release + strip`), saves 50 bytes per `event!`.

Test environment:
```
toolchain: nightly-aarch64-apple-darwin
rustc-version: rustc 1.70.0-nightly (88fb1b922 2023-04-10)
```

There are also performance improvements in the benchmarks:

```
event/scoped [-40.689% -40.475% -40.228%]
event/scoped_recording [-14.972% -14.685% -14.410%]
event/global [-48.412% -48.217% -48.010%]
span_fields/scoped [-25.317% -24.876% -24.494%]
span_fields/global [-39.695% -39.488% -39.242%]
span_repeated/global [-27.514% -26.633% -25.298%]
static/baseline_single_threaded [-32.275% -32.032% -31.808%]
static/single_threaded [-29.628% -29.376% -29.156%]
static/enabled_one [-29.777% -29.544% -29.305%]
static/enabled_many [-30.901% -30.504% -30.140%]
dynamic/baseline_single_threaded [-20.157% -19.880% -19.603%]
```

I retried benchmark several times and the improvements seem to be fairly
stable.

raw log: https://gist.github.com/ldm0/6573935f4979d2645fbcf5bde7361386

tracing-core/src/field.rs

@@ -820,6 +820,7 @@ impl FieldSet {
     ///
     /// [`Identifier`]: super::callsite::Identifier
     /// [`Callsite`]: super::callsite::Callsite
+    #[inline]
     pub(crate) fn callsite(&self) -> callsite::Identifier {
         callsite::Identifier(self.callsite.0)
     }
@@ -857,6 +858,7 @@ impl FieldSet {
     }
 
     /// Returns an iterator over the `Field`s in this `FieldSet`.
+    #[inline]
     pub fn iter(&self) -> Iter {
         let idxs = 0..self.len();
         Iter {
@@ -960,6 +962,7 @@ impl PartialEq for FieldSet {
 
 impl Iterator for Iter {
     type Item = Field;
+    #[inline]
     fn next(&mut self) -> Option<Field> {
         let i = self.idxs.next()?;
         Some(Field {

tracing-core/src/metadata.rs

@@ -273,6 +273,7 @@ impl<'a> Metadata<'a> {
     }
 
     /// Returns the names of the fields on the described span or event.
+    #[inline]
     pub fn fields(&self) -> &field::FieldSet {
         &self.fields
     }

tracing/src/macros.rs

@@ -2311,7 +2311,7 @@ macro_rules! valueset {
         )
     };
 
-    // Remainder is unparseable, but exists --- must be format args!
+    // Remainder is unparsable, but exists --- must be format args!
     (@ { $(,)* $($out:expr),* }, $next:expr, $($rest:tt)+) => {
         $crate::valueset!(@ { (&$next, Some(&format_args!($($rest)+) as &dyn Value)), $($out),* }, $next, )
     };