LogDog: Implement prefix registration in Butler.

client/cmd/logdog_butler/output_log.go

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package main
 
 import (
         "github.com/luci/luci-go/client/internal/logdog/butler/output"
         logOutput "github.com/luci/luci-go/client/internal/logdog/butler/output/log"
         "github.com/luci/luci-go/common/flag/multiflag"
@@ skipping 11 matching lines @@
         flags := opt.Flags()
         flags.IntVar(&f.bundleSize, "bundle-size", 1024*1024,
                 "Maximum bundle size.")
 
         return opt
 }
 
 func (f *logOutputFactory) configOutput(a *application) (output.Output, error) {
         return logOutput.New(a, f.bundleSize), nil
 }
+
+func (f *logOutputFactory) scopes() []string { return nil }