add Write method to config source

config/source/flag/flag.go

@@ -77,6 +77,10 @@ func (fs *flagsrc) Watch() (source.Watcher, error) {
 	return source.NewNoopWatcher()
 }
 
+func (fs *flagsrc) Write(cs *source.ChangeSet) error {
+	return nil
+}
+
 func (fs *flagsrc) String() string {
 	return "flag"
 }