Currently 1.2.0 I don't think it's possible to get a document's metadata keys from the PD model.
It is possible to get the metadata keys by calling PDDocumentInformation.getDictionary().keySet() and calling getName() on the COSName objects returned.
But that means having to know about the COSDictionary and COSName objects i.e. the lower-level COS model.
It would be nice to be able to retrieve the metadata keys directly from the PD model.
I'm about to add a patch that does this. The patch will also add a new test class.