Appender-Tracing - extend spl treatment of message when recording str (#2689)

Co-authored-by: Lalit Kumar Bhasin <lalit_fin@yahoo.com>

opentelemetry-appender-tracing/CHANGELOG.md

@@ -5,6 +5,32 @@
 Fixes [1682](https://github.com/open-telemetry/opentelemetry-rust/issues/1682).
 "spec_unstable_logs_enabled" feature now do not suppress logs for other layers.
 
+The special treatment of the "message" field has been extended when recording
+string values. With this change, when a log is emitted with a field named
+"message" (and string value), its value is directly assigned to the LogRecord’s
+body rather than being stored as an attribute named "message". This offers a
+slight performance improvement over previous.
+
+For example, the below will now produce LogRecord with the message value
+populated as LogRecord's body:
+
+```rust
+error!(name: "my-event-name", target: "my-system", event_id = 20, user_name = "otel", user_email = "otel@opentelemetry.io", message = "This is an example message");
+```
+
+Previously, Body was only populated when the below style was used.
+
+```rust
+error!(name: "my-event-name", target: "my-system", event_id = 20, user_name = "otel", user_email = "otel@opentelemetry.io", "This is an example message");
+```
+
+This style, while slightly slower, should still be used when the value is not a
+simple string, but require format arguments as in the below example.
+
+```rust
+error!(name: "my-event-name", target: "my-system", event_id = 20, user_name = "otel", user_email = "otel@opentelemetry.io", "This is an example message with format arguments {} and {}", "foo", "bar");
+```
+
 ## 0.28.1
 
 Released 2025-Feb-12

opentelemetry-appender-tracing/src/layer.rs

@@ -88,15 +88,14 @@ impl<LR: LogRecord> tracing::field::Visit for EventVisitor<'_, LR> {
         if is_duplicated_metadata(field.name()) {
             return;
         }
-        //TODO: Consider special casing "message" to populate body and document
-        // to users to use message field for log message, to avoid going to the
-        // record_debug, which has dyn dispatch, string allocation and
-        // formatting cost.
-
         //TODO: Fix heap allocation. Check if lifetime of &str can be used
         // to optimize sync exporter scenario.
-        self.log_record
-            .add_attribute(Key::new(field.name()), AnyValue::from(value.to_owned()));
+        if field.name() == "message" {
+            self.log_record.set_body(AnyValue::from(value.to_owned()));
+        } else {
+            self.log_record
+                .add_attribute(Key::new(field.name()), AnyValue::from(value.to_owned()));
+        }
     }
 
     fn record_bool(&mut self, field: &tracing_core::Field, value: bool) {