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

Make the key italic by default.

examples/direct_logger.rs

@@ -8,6 +8,8 @@ use env_logger::{Builder, WriteStyle};
 
 use log::{Level, LevelFilter, Log, MetadataBuilder, Record};
 
+static kvs: (&str, &str) = ("test", "something");
+
 fn record() -> Record<'static> {
     let error_metadata = MetadataBuilder::new()
         .target("myApp")
@@ -20,6 +22,7 @@ fn record() -> Record<'static> {
         .line(Some(433))
         .file(Some("app.rs"))
         .module_path(Some("server"))
+        .key_values(&kvs)
         .build()
 }
 

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.