LogDog: Implement prefix registration in Butler.

client/internal/logdog/butler/output/pubsub/pubsubOutput.go

Index: client/internal/logdog/butler/output/pubsub/pubsubOutput.go
diff --git a/client/internal/logdog/butler/output/pubsub/pubsubOutput.go b/client/internal/logdog/butler/output/pubsub/pubsubOutput.go
index 9b9c691a021d6b1ba0ebfd88207ea8f52f1f7145..5801aa7b005e5e4d98ffb0c8b69bd80518014b59 100644
--- a/client/internal/logdog/butler/output/pubsub/pubsubOutput.go
+++ b/client/internal/logdog/butler/output/pubsub/pubsubOutput.go
@@ -14,6 +14,7 @@ import (
 	"github.com/luci/luci-go/client/internal/logdog/butler/output"
 	gcps "github.com/luci/luci-go/common/gcloud/pubsub"
 	"github.com/luci/luci-go/common/logdog/butlerproto"
+	"github.com/luci/luci-go/common/logdog/types"
 	log "github.com/luci/luci-go/common/logging"
 	"github.com/luci/luci-go/common/proto/logdog/logpb"
 	"github.com/luci/luci-go/common/recordio"
@@ -40,6 +41,9 @@ type Config struct {
 	// Topic is the Pub/Sub topic to publish to.
 	Topic Topic
 
+	// Secret, if not nil, is the prefix secret to attach to each outgoing bundle.
+	Secret types.PrefixSecret
+
 	// Compress, if true, enables zlib compression.
 	Compress bool
 
@@ -96,6 +100,7 @@ func (o *pubSubOutput) SendBundle(bundle *logpb.ButlerLogBundle) error {
 	b := o.bufferPool.Get().(*buffer)
 	defer o.bufferPool.Put(b)
 
+	bundle.Secret = []byte(o.Secret)
 	message, err := o.buildMessage(b, bundle)
 	if err != nil {
 		log.Fields{