Warn when preserve is true and the original would be overwritten

plugin/helper/parser.go

@@ -2,6 +2,7 @@ package helper
 
 import (
 	"context"
+	"reflect"
 
 	"github.com/observiq/carbon/entry"
 	"github.com/observiq/carbon/errors"
@@ -34,6 +35,14 @@ func (c ParserConfig) Build(context plugin.BuildContext) (ParserPlugin, error) {
 		c.ParseTo.FieldInterface = entry.NewRecordField()
 	}
 
+	if reflect.DeepEqual(c.ParseFrom, c.ParseTo) && c.Preserve {
+		transformerPlugin.Warnw(
+			"preserve is true, but parse_to is set to the same field as parse_from, "+
+				"which will cause the original value to be overwritten",
+			"plugin_id", c.PluginID,
+		)
+	}
+
 	parserPlugin := ParserPlugin{
 		TransformerPlugin: transformerPlugin,
 		ParseFrom:         c.ParseFrom,