Add a filter to control what metadata is cloned

The current version of this plugin has a filter in-place to filter the post data, but it doesn't have a way to filter out the post's metadata that is being inserted afterward.

It would be helpful if it were possible to filter out the metadata, as well. This would allow someone who has a custom post type with custom post meta to modify the metadata for the scheduled post if-needed.

Start by locating the existing filter for post data and the code path that inserts post metadata afterward. Add the corresponding metadata filtering entry point so custom post metadata can be modified or excluded for scheduled posts, then verify that metadata filtering works without changing the existing post-data filter behavior.

索引モデルが issue の本文から書いたものです。