Fix default metadata namespaces (#1633)

Summary: #1368 added the ability to limit the metadata service to
specific namespaces. By default, if no namespaces are mentioned, the
flag is supposed to choose all namespaces.
However, if the environment variable is not specified, it is actually
parsed as an empty list and the default is not properly chosen. Instead,
we can assume no users actually want to watch 0 namespaces, and set it
to all namespaces when that occurs.

Relevant Issues: N/A

Type of change: /kind bug

Test Plan: Skaffold deploy metadata service

Signed-off-by: Michelle Nguyen <michellenguyen@pixielabs.ai>

Author: aimichelle

src/vizier/services/metadata/metadata_server.go

@@ -238,6 +238,9 @@ func main() {
 	mdh := k8smeta.NewHandler(updateCh, k8sMds, k8sMds, nc)
 
 	namespaces := viper.GetStringSlice("metadata_namespaces")
+	if len(namespaces) == 0 {
+		namespaces = []string{v1.NamespaceAll}
+	}
 	k8sMc, err := k8smeta.NewController(namespaces, updateCh)
 	defer k8sMc.Stop()