type-safe metadata protocol

[d-v-b]

To make our to-json and from-json serialization type-safe, we need a class like this:

class Metadata(Generic[FromType, ToType]):

  @classmethod
  def from_json(cls, data: FromType) -> Self:
    ...

  def to_json(self) -> ToType:
    ...

This violates liskov because FromType gets narrower in subclasses. ToType and FromType effectively impose opposite variance requirements on Metadata here.

But it does work if we use generic protocols instead! So that's what we should do. I think it would be really nice to make all of our JSON serialization type-safe this way.

[d-v-b]

here's a fleshed out example shape for a serializable object:

from typing import Protocol, TypeVar, Self

T_contra = TypeVar("T_contra", contravariant=True)
T_co = TypeVar("T_co", covariant=True)

class Serializable(Protocol[T_contra, T_co]):
    @classmethod
    def from_json(cls, data: T_contra) -> Self: ...

    @classmethod
    def try_from_json(cls, data: object) -> Self: ...

    def to_json(self) -> T_co: ...

I feel some urgency to improve our metadata serialization as we consider adding new chunk grids and codecs. So I might put together a PR that adds this protocol, or something like it, soon.