Add HTTP User Agent flag for cipd.

cipd/client/cmd/cipd/main.go

Index: cipd/client/cmd/cipd/main.go
diff --git a/cipd/client/cmd/cipd/main.go b/cipd/client/cmd/cipd/main.go
index a94381f4d331650370cb23342c0a927426b71be5..bfad54021d40538197958fbb828dd9b0148cb759 100644
--- a/cipd/client/cmd/cipd/main.go
+++ b/cipd/client/cmd/cipd/main.go
@@ -235,10 +235,13 @@ type ClientOptions struct {
 	authFlags  authcli.Flags
 	serviceURL string
 	cacheDir   string
+	userAgent  string
 }
 
 func (opts *ClientOptions) registerFlags(f *flag.FlagSet) {
 	f.StringVar(&opts.serviceURL, "service-url", "", "URL of a backend to use instead of the default one.")
+	f.StringVar(&opts.userAgent, "http-user-agent", "",
+		"User Agent text to add. If specified, the UA will be '<this_option>/"+cipd.UserAgent+"'.")
 	f.StringVar(&opts.cacheDir, "cache-dir", "", "Directory for shared cache")
 	opts.authFlags.Register(f, auth.Options{})
 }
@@ -252,9 +255,14 @@ func (opts *ClientOptions) makeCipdClient(ctx context.Context, root string) (cip
 	if err != nil {
 		return nil, err
 	}
+	ua := cipd.UserAgent
+	if opts.userAgent != "" {
+		ua = fmt.Sprintf("%s/%s", opts.userAgent, ua)
+	}
 	return cipd.NewClient(cipd.ClientOptions{
 		ServiceURL:          opts.serviceURL,
 		Root:                root,
+		UserAgent:           ua,
 		CacheDir:            opts.cacheDir,
 		AuthenticatedClient: client,
 		AnonymousClient:     http.DefaultClient,