EmbeddedDocumentField does not properly merge meta if it already exists

For a project I am working on, I need both the merging of the meta dict and merging with the parent's meta dict (I am using a custom field in meta, coupled with inheritance on the EmbeddedDocuments themselves). However, this is not working properly. After quite a bit of searching, I traced it back to here. Note that there's no merging performed if the subclass specifies it's own meta. That meta is simply copied over to _meta, which means we lose allow_inheritance (which means we further lose _cls when saving EmbeddedDocument instances).

I am unsure of how to proceed here, but would be happy to open a PR. However, I don't know whether an unconditional merge of these two dictionaries is beneficial to all scenarios. If anyone has feedback on how to move forward here, please reply.

I am currently working around this by manually specifying the allow_inheritance field on the child classes. If any of this is unclear, please let me know and I will work up a small example.

Start in mongoengine/base/metaclasses.py at lines 34-47 and trace how EmbeddedDocument subclasses copy or merge meta. Build the small inheritance example mentioned in the issue, then verify how allow_inheritance and _cls behave when saving instances. Done means the intended parent and child metadata are preserved without breaking existing scenarios.