fix #823 : lower JsonExtract debug (#824)

* lower key not found log level, fix #823

pkg/exprhelpers/jsonextract.go

@@ -13,7 +13,12 @@ func JsonExtractLib(jsblob string, target ...string) string {
 		jsonparser.StringToBytes(jsblob),
 		target...,
 	)
+
 	if err != nil {
+		if err == jsonparser.KeyPathNotFoundError {
+			log.Debugf("%+v doesn't exist", target)
+			return ""
+		}
 		log.Errorf("jsonExtractLib : %+v : %s", target, err)
 		return ""
 	}
@@ -22,7 +27,6 @@ func JsonExtractLib(jsblob string, target ...string) string {
 		return ""
 	}
 	strvalue := string(value)
-	//debug stuff
 	return strvalue
 }