feat: allow changing the casing of the Level field

Users can now choose between `Lowercase` and `Uppercase` for the
casing of the Level fields value.

Example output (pretty-printed for readability):

`Casing::Uppercase`

```json
{
  "severity": "ERROR",
  "ts": 1696892235,
  "target": "tracing_ndjson::tests",
  "fields": {
    "msg": "hello world from error"
  }
}
```

The default remains `Casing::Lowercase`.

src/formatter.rs

@@ -12,6 +12,7 @@ use crate::{visitor, Error};
 
 const DEFAULT_TIMESTAMP_FORMAT: crate::TimestampFormat = crate::TimestampFormat::Rfc3339;
 const DEFAULT_LEVEL_NAME: &str = "level";
+const DEFAULT_LEVEL_VALUE_CASING: crate::Casing = crate::Casing::Lowercase;
 const DEFAULT_MESSAGE_NAME: &str = "message";
 const DEFAULT_TARGET_NAME: &str = "target";
 const DEFAULT_TIMESTAMP_NAME: &str = "timestamp";
@@ -21,6 +22,7 @@ const DEFAULT_FLATTEN_FIELDS: bool = true;
 /// This is used to format the event field in the JSON output.
 pub struct JsonEventFormatter {
     level_name: &'static str,
+    level_value_casing: crate::Casing,
     message_name: &'static str,
     target_name: &'static str,
     timestamp_name: &'static str,
@@ -32,6 +34,7 @@ impl Default for JsonEventFormatter {
     fn default() -> Self {
         Self {
             level_name: DEFAULT_LEVEL_NAME,
+            level_value_casing: DEFAULT_LEVEL_VALUE_CASING,
             message_name: DEFAULT_MESSAGE_NAME,
             target_name: DEFAULT_TARGET_NAME,
             timestamp_name: DEFAULT_TIMESTAMP_NAME,
@@ -51,6 +54,11 @@ impl JsonEventFormatter {
         self
     }
 
+    pub fn with_level_value_casing(mut self, level_value_casing: crate::Casing) -> Self {
+        self.level_value_casing = level_value_casing;
+        self
+    }
+
     pub fn with_message_name(mut self, message_name: &'static str) -> Self {
         self.message_name = message_name;
         self
@@ -93,11 +101,13 @@ where
         let mut binding = serde_json::Serializer::new(Vec::new());
         let mut serializer = binding.serialize_map(None).map_err(Error::Serde)?;
 
+        let level_str = match self.level_value_casing {
+            crate::Casing::Lowercase => event.metadata().level().to_string().to_lowercase(),
+            crate::Casing::Uppercase => event.metadata().level().to_string().to_uppercase(),
+        };
+
         serializer
-            .serialize_entry(
-                self.level_name,
-                &event.metadata().level().to_string().to_lowercase(),
-            )
+            .serialize_entry(self.level_name, &level_str)
             .map_err(Error::Serde)?;
 
         if matches!(

src/lib.rs

@@ -90,6 +90,11 @@ impl TimestampFormat {
     }
 }
 
+pub enum Casing {
+    Lowercase,
+    Uppercase,
+}
+
 #[derive(Debug, thiserror::Error)]
 enum Error {
     #[error("fmt error: {0}")]
@@ -110,6 +115,7 @@ impl From<Error> for std::fmt::Error {
 /// This is used to configure the JSON formatter.
 /// The default configuration is:
 /// * level_name: "level"
+/// * level_value_casing: Casing::Lowercase
 /// * message_name: "message"
 /// * target_name: "target"
 /// * timestamp_name: "timestamp"
@@ -125,6 +131,7 @@ impl From<Error> for std::fmt::Error {
 ///     .with(
 ///         tracing_ndjson::Builder::default()
 ///             .with_level_name("severity")
+///            .with_level_value_casing(tracing_ndjson::Casing::Uppercase)
 ///             .with_message_name("msg")
 ///             .with_timestamp_name("ts")
 ///             .with_timestamp_format(tracing_ndjson::TimestampFormat::Unix)
@@ -166,6 +173,13 @@ impl Builder {
         self
     }
 
+    /// Set the casing for the level field value.
+    /// The default is Casing::Lowercase.
+    pub fn with_level_value_casing(mut self, casing: Casing) -> Self {
+        self.events = self.events.with_level_value_casing(casing);
+        self
+    }
+
     /// Set the field name for the message field.
     /// The default is "message".
     pub fn with_message_name(mut self, message_name: &'static str) -> Self {
@@ -277,6 +291,7 @@ mod tests {
         let subscriber = tracing_subscriber::registry().with(
             builder()
                 .with_level_name("severity")
+                .with_level_value_casing(Casing::Uppercase)
                 .with_message_name("msg")
                 .with_timestamp_name("ts")
                 .with_timestamp_format(TimestampFormat::Unix)