Keep observer composition storage proportional to selected callbacks

internal/observation/compose.go

@@ -2,15 +2,26 @@ package observation
 
 // Compose copies and synchronously invokes non-nil observers in order.
 func Compose[T any, F ~func(T)](observers ...F) F {
-	selected := make([]F, 0, len(observers))
+	count := 0
+	var single F
+	for _, observer := range observers {
+		if observer != nil {
+			count++
+			single = observer
+		}
+	}
+	if count == 0 {
+		return nil
+	}
+	if count == 1 {
+		return single
+	}
+	selected := make([]F, 0, count)
 	for _, observer := range observers {
 		if observer != nil {
 			selected = append(selected, observer)
 		}
 	}
-	if len(selected) == 0 {
-		return nil
-	}
 	return func(event T) {
 		for _, observer := range selected {
 			observer(event)