internal/metadata.go
func mustGetMetadata(key string) []byte {
	b, err := getMetadata(key)
	if err != nil {
		panic(fmt.Sprintf("Metadata fetch failed for '%s': %v", key, err))
	}
	return b
}
func mustGetMetadata(key string) []byte {
	b, err := getMetadata(key)
	if err != nil {
		panic(fmt.Sprintf("Metadata fetch failed for '%s': %v", key, err))
	}
	return b
}
// TODO(dsymonds): Do we need to support default values, like Python?