fix: remove LLMMetricAnnotation from response stream (#1499)

Signed-off-by: Hongkuan Zhou <tedzhouhk@gmail.com>

lib/llm/src/http/service/openai.rs

@@ -499,12 +499,19 @@ fn process_event_converter<T: Serialize>(
     annotated: EventConverter<T>,
     response_collector: &mut ResponseMetricCollector,
 ) -> Result<Event, axum::Error> {
-    let annotated = annotated.0;
+    let mut annotated = annotated.0;
 
     // update metrics
     if let Ok(Some(metrics)) = LLMMetricAnnotation::from_annotation(&annotated) {
         response_collector.observe_current_osl(metrics.output_tokens);
         response_collector.observe_response(metrics.input_tokens, metrics.chunk_tokens);
+
+        // Chomp the LLMMetricAnnotation so it's not returned in the response stream
+        // TODO: add a flag to control what is returned in the SSE stream
+        if annotated.event.as_deref() == Some(crate::preprocessor::ANNOTATION_LLM_METRICS) {
+            annotated.event = None;
+            annotated.comment = None;
+        }
     }
 
     let mut event = Event::default();