code: public static Metadata of(Map<String, String> values) {
      return newBuilder().setValues(values).build();
    }
docs: Returns a {@code Metadata} object given the the metadata as a map. The total size of all keys
  and values must be less than 512 KB. Keys must conform to the following regexp: {@code
  [a-zA-Z0-9-_]+}, and be less than 128 bytes in length. This is reflected as part of a URL in
  the metadata server. Additionally, to avoid ambiguity, keys must not conflict with any other
  metadata keys for the project. Values must be less than or equal to 32768 bytes.