Refactor: collect possible keys of Document `_metadata` and store globally

[JohannesMessner]

In many places (such as, e.g., here), we use string keys to access information in a Document's _metadata.

This is not very clean and prone to bugs/typos/etc. We should collect these keys and store them as a global constant, or similar.

This might potentially also affect other structures that we maintain, not only _metadata.