Additional custom metadata for module

[xlc]

It will be useful to have decl_module takes an additional ModuleMetadata Encodeable and have it stored and passed via metadata rpc call.

This can be useful for various purposes, such as a workaround of paritytech/polkadot-sdk#368 (manually encode the child storage field info, and then JS SDK can decode it and extract the child storage details).

This is like a more generalised version of the constants field in the metadata, that it is not used in runtime, but significantly improves SDK developer's life quality.

[shawntabrizi]

Should ideally be supported with some expansion of: #5678

[apopiak]

related to #917

[gui1117]

with pallet macro there is extra constants: https://substrate.dev/rustdocs/v3.0.0/frame_support/attr.pallet.html#extra-constants-palletextra_constants-optional

it can add any values to the metadata under any names, would that be enough ?
what more would be needed.

[xlc]

Never aware of this feature but I think it solves my issue

[shawntabrizi]

I don't get it. How will this be used?

[xlc]

with extra_constants, I should be able to define arbitrary constants that is accessible via metadata, so it can expose things like non-runtime defined parameters, child storage key prefix or any other metadata that could be used by SDK. If I want, I should be able to have it return a JSON string represents custom types and partially address #917.