Add Butler global tags.

logdog/client/butler/butler.go

Index: logdog/client/butler/butler.go
diff --git a/logdog/client/butler/butler.go b/logdog/client/butler/butler.go
index 80b0d04cf94be17b921e34e2b8811c5a6f89f7b5..f8a1d134fcd77ffbb5410fb03018a9b478255ef7 100644
--- a/logdog/client/butler/butler.go
+++ b/logdog/client/butler/butler.go
@@ -25,6 +25,7 @@ import (
 	"github.com/luci/luci-go/logdog/client/butler/streamserver"
 	"github.com/luci/luci-go/logdog/client/butlerlib/streamproto"
 	"github.com/luci/luci-go/logdog/common/types"
+
 	"golang.org/x/net/context"
 )
 
@@ -56,6 +57,11 @@ type Config struct {
 	// Prefix is the log stream common prefix value.
 	Prefix types.StreamName
 
+	// GlobalTags are a set of global log stream tags to apply to individual
+	// streams on registration. Individual stream tags will override tags with
+	// the same key.
+	GlobalTags streamproto.TagMap
+
 	// BufferLogs, if true, instructs the butler to buffer collected log data
 	// before sending it to Output.
 	BufferLogs bool
@@ -398,6 +404,17 @@ func (b *Butler) AddStream(rc io.ReadCloser, p *streamproto.Properties) error {
 		return err
 	}
 
+	// Build per-stream tag map.
+	if l := len(b.c.GlobalTags); l > 0 {
+		for k, v := range b.c.GlobalTags {
+			// Add only global flags that aren't already present (overridden) in
+			// stream tags.
+			if _, ok := p.Tags[k]; !ok {
+				p.Tags[k] = v
+			}
+		}
+	}
+
 	if p.Timeout > 0 {
 		if rts, ok := rc.(iotools.ReadTimeoutSetter); ok {
 			if err := rts.SetReadTimeout(p.Timeout); err != nil {