Built-in and user-defined metadata store API

[yfukai]

It may be useful to have a common API to store global attributes for the graph. For example, a graph can have

• Read-only built-in attributes for the graph schema: default values and dtypes of the node and edge attributes, compression of the array etc.
• Writable user-defined attributes for arbitrary metadata for the graph. For example, the software that generated the graph etc.

[JoOkuma]

Hi @yfukai, can you describe how this differs from or complements the existing metadata field?

[yfukai]

Thanks @JoOkuma, I think it's better to have a spec for private metadata, that are not writable, to store the column information such as dtype. So the implementation is done but we need to decide the schema (default metadata names) and a trick to make it not read-only.