feat(kv): Add styling for key in default format

Make the key italic by default.

examples/direct_logger.rs

@@ -8,19 +8,27 @@ use env_logger::{Builder, WriteStyle};
 
 use log::{Level, LevelFilter, Log, MetadataBuilder, Record};
 
+#[cfg(feature = "unstable-kv")]
+static KVS: (&str, &str) = ("test", "something");
+
 fn record() -> Record<'static> {
     let error_metadata = MetadataBuilder::new()
         .target("myApp")
         .level(Level::Error)
         .build();
 
-    Record::builder()
+    let mut builder = Record::builder();
+    builder
         .metadata(error_metadata)
         .args(format_args!("Error!"))
         .line(Some(433))
         .file(Some("app.rs"))
-        .module_path(Some("server"))
-        .build()
+        .module_path(Some("server"));
+    #[cfg(feature = "unstable-kv")]
+    {
+        builder.key_values(&KVS);
+    }
+    builder.build()
 }
 
 fn main() {

src/fmt/kv.rs

@@ -1,6 +1,7 @@
 use std::io::{self, Write};
 
-use super::Formatter;
+use super::{Formatter, StyledValue, WriteStyle};
+use anstyle::Style;
 use log::kv::{source::Source, Error, Key, Value, Visitor};
 
 /// Format function for serializing key/value pairs
@@ -41,7 +42,27 @@ impl<'a, 'kvs> Visitor<'kvs> for DefaultVisitor<'a> {
     fn visit_pair(&mut self, key: Key, value: Value<'kvs>) -> Result<(), Error> {
         // TODO: add styling
         // tracing-subscriber uses italic for the key and dimmed for the =
-        write!(self.0, " {}={}", key, value)?;
+        write!(self.0, " {}={}", self.style_key(key), value)?;
         Ok(())
     }
 }
+
+impl DefaultVisitor<'_> {
+    fn style_key<'k>(&self, text: Key<'k>) -> StyledValue<Key<'k>> {
+        #[cfg(feature = "color")]
+        {
+            StyledValue {
+                style: if self.0.write_style == WriteStyle::Never {
+                    Style::new()
+                } else {
+                    Style::new().italic()
+                },
+                value: text,
+            }
+        }
+        #[cfg(not(feature = "color"))]
+        {
+            key
+        }
+    }
+}

src/fmt/mod.rs

@@ -292,6 +292,9 @@ impl<T: std::fmt::Display> std::fmt::Display for StyledValue<T> {
     }
 }
 
+#[cfg(not(feature = "color"))]
+type StyledValue<T> = T;
+
 /// The default format.
 ///
 /// This format needs to work with any combination of crate features.

src/logger.rs

@@ -321,7 +321,7 @@ impl Builder {
     /// The format function is expected to output the string directly to the `Formatter` so that
     /// implementations can use the [`std::fmt`] macros, similar to the main format function.
     ///
-    /// The default format uses a space to separte each key-value pair, with an "=" between
+    /// The default format uses a space to separate each key-value pair, with an "=" between
     /// the key and value.
     #[cfg(feature = "unstable-kv")]
     pub fn format_key_values<F: 'static>(&mut self, format: F) -> &mut Self