[pkg/ottl] Support for append

pkg/ottl/ottlfuncs/README.md

@@ -63,10 +63,10 @@ Available Editors:
 
 `append(target, Optional[value], Optional[values])`
 
-The `append` function appens single or multiple string values to target. 
-`append` converts scalar values into an array if the field exists but is not an array and creates an array containing the provided values if the field doesn’t exist.
+The `append` function appends single or multiple string values to `target`. 
+`append` converts scalar values into an array if the field exists but is not an array, and creates an array containing the provided values if the field doesn’t exist.
 
-Resulting field is always of type `pcommon.Slice` keeping types of existing values appending values as set not performing any conversions.
+Resulting field is always of type `pcommon.Slice` and will not convert the types of existing or new items in the slice.
 
   - `append(attributes["tags"], "prod")`
   - `append(attributes["tags"], values = ["staging", "staging:east"])`

pkg/ottl/ottlfuncs/func_append.go

@@ -32,7 +32,7 @@ func createAppendFunction[K any](_ ottl.FunctionContext, oArgs ottl.Arguments) (
 
 func appendTo[K any](target ottl.GetSetter[K], value ottl.Optional[ottl.Getter[K]], values ottl.Optional[[]ottl.Getter[K]]) (ottl.ExprFunc[K], error) {
 	if value.IsEmpty() && values.IsEmpty() {
-		return nil, fmt.Errorf("at least one of the optional arguments must be provided be provided")
+		return nil, fmt.Errorf("at least one of the optional arguments ('value' or 'values') must be provided")
 	}
 
 	return func(ctx context.Context, tCtx K) (any, error) {
@@ -63,7 +63,7 @@ func appendTo[K any](target ottl.GetSetter[K], value ottl.Optional[ottl.Getter[K
 				case pcommon.ValueTypeSlice:
 					res = append(res, targetType.Slice().AsRaw()...)
 				default:
-					return nil, fmt.Errorf("unsupported type of target field")
+					return nil, fmt.Errorf("unsupported type of target field: %q", targetType.Type())
 				}
 
 			case []string:
@@ -88,7 +88,7 @@ func appendTo[K any](target ottl.GetSetter[K], value ottl.Optional[ottl.Getter[K
 			case any:
 				res = append(res, targetType)
 			default:
-				return nil, fmt.Errorf("unsupported type of target field")
+				return nil, fmt.Errorf("unsupported type of target field: '%T'", t)
 			}
 		}