LogDog: Implement prefix registration in Butler.

client/cmd/logdog_butler/output.go

Index: client/cmd/logdog_butler/output.go
diff --git a/client/cmd/logdog_butler/output.go b/client/cmd/logdog_butler/output.go
index 9d896a58619007db70999bd50a759803597c3416..7c5911788d7d61b689e4a103115ead7832ba3f5f 100644
--- a/client/cmd/logdog_butler/output.go
+++ b/client/cmd/logdog_butler/output.go
@@ -5,6 +5,8 @@
 package main
 
 import (
+	"sort"
+
 	"github.com/luci/luci-go/client/internal/logdog/butler/output"
 	"github.com/luci/luci-go/common/flag/multiflag"
 )
@@ -12,6 +14,7 @@ import (
 type outputFactory interface {
 	option() multiflag.Option
 	configOutput(a *application) (output.Output, error)
+	scopes() []string
 }
 
 // outputConfigFlag instance that produces a MessageOutput instance when run.
@@ -70,3 +73,19 @@ func registerOutputFactory(f outputFactory) {
 func getOutputFactories() []outputFactory {
 	return outputFactories
 }
+
+func allOutputScopes() []string {
+	scopes := make(map[string]struct{})
+	for _, of := range outputFactories {
+		for _, scope := range of.scopes() {
+			scopes[scope] = struct{}{}
+		}
+	}
+
+	allScopes := make([]string, 0, len(scopes))
+	for scope := range scopes {
+		allScopes = append(allScopes, scope)
+	}
+	sort.Strings(allScopes)
+	return allScopes
+}