Butler: fix global tags applied to empty map.

logdog/client/butler/bundler/bundler.go

Index: logdog/client/butler/bundler/bundler.go
diff --git a/logdog/client/butler/bundler/bundler.go b/logdog/client/butler/bundler/bundler.go
index eaff529d4062a5fc8a3434a770616d2229a80255..f6cb3e9f3a7f63ed6836ff8757aea05524b20a4f 100644
--- a/logdog/client/butler/bundler/bundler.go
+++ b/logdog/client/butler/bundler/bundler.go
@@ -52,6 +52,7 @@ type bundlerStream interface {
 	name() string
 	expireTime() (time.Time, bool)
 	nextBundleEntry(*builder, bool) bool
+	streamDesc() *logpb.LogStreamDescriptor
 }
 
 // Bundler is the main Bundler instance. It exposes goroutine-safe endpoints for
@@ -141,6 +142,26 @@ func (b *Bundler) Register(p *streamproto.Properties) (Stream, error) {
 	return s, nil
 }
 
+// GetStreamDescs returns the set of registered stream names mapped to their
+// descriptors.
+//
+// This is intended for testing purposes. DO NOT modify the resulting
+// descriptors.
+func (b *Bundler) GetStreamDescs() map[string]*logpb.LogStreamDescriptor {
+	b.streamsLock.Lock()
+	defer b.streamsLock.Unlock()
+
+	if len(b.streams) == 0 {
+		return nil
+	}
+
+	streams := make(map[string]*logpb.LogStreamDescriptor, len(b.streams))
+	for k, s := range b.streams {
+		streams[k] = s.streamDesc()
+	}
+	return streams
+}
+
 // CloseAndFlush closes the Bundler, alerting it that no more streams will be
 // added and that existing data may be aggressively output.
 //