Problem

Currently, when serializing a Paginator, the pagination metadata is always output under the hardcoded key "metadata":

{
  "data": [...],
  "metadata": {  // always hardcoded
  ...
  }
}

This key cannot be changed, which is problematic because:

• Internal API conventions — teams that have already established a different key (e.g. "meta", "pagination") across their API cannot adopt this library without breaking their existing contract with clients.
• Backward compatibility — projects migrating to http-transformers from another serialization layer are forced to rename the key in all their API responses, which may break existing clients.

Proposed solution

Add a MetadataKey property to BaseSerializer, mirroring the existing wrap property, with a default value of "metadata".
The Wrappers generic should be extended with a MetadataKey type parameter so that the output type is fully inferred:

class ApiSerializer extends BaseSerializer<{
  Wrap: 'data'
  MetadataKey: 'meta'   
  PaginationMetaData: SimplePaginatorMetaKeys
}> {
  wrap: 'data' = 'data'
  metadataKey: 'meta' = 'meta'
  // ...
}

Happy to open a PR with these changes if the idea sounds good to you !
The transformer pattern is really a good idea ! 🙂