KV Key metadata types to narrow

[ckohen]

Between version 3.2.0 and 3.3.0 the type for KV's Key['metadata'] (and MetadataGetResponse) was changed from unknown to Record<string, unknown>.

This type is too narrow, as the metadata can be set to any valid JSON string. Since the value of the metadata is updated as a raw JSON string, this arbitrary nature is very useable. Presumably this is parsed by JSON.parse which respects this behavior. This can either be reverted to unknown, or widened to be a JSONValue type that is essentially most primitives.