deprecate enable-managedby-label flag in favor of a field

kustomize/commands/build/build.go

@@ -104,6 +104,8 @@ func NewCmdBuild(
 	AddFlagEnablePlugins(cmd.Flags())
 	AddFlagReorderOutput(cmd.Flags())
 	AddFlagEnableManagedbyLabel(cmd.Flags())
+	cmd.Flags().MarkDeprecated(managedByFlag,
+		"The flag `enable-managedby-label` has been deprecated. Use the `managedByLabel` option in the `buildMetadata` field instead.")
 	AddFlagEnableHelm(cmd.Flags())
 	return cmd
 }

kustomize/commands/build/flagaddmanagedby.go

@@ -10,10 +10,12 @@ import (
 	"sigs.k8s.io/kustomize/api/konfig"
 )
 
+const managedByFlag = "enable-managedby-label"
+
 func AddFlagEnableManagedbyLabel(set *pflag.FlagSet) {
 	set.BoolVar(
 		&theFlags.enable.managedByLabel,
-		"enable-managedby-label",
+		managedByFlag,
 		false,
 		`enable adding `+konfig.ManagedbyLabelKey)
 }