DocumentIdMapping#numericField() is missing.

The problem here is that NumericFieldMapping extends FieldMapping which adds itself as a field to the current property. Therefore the check protecting against reconfiguration of the implicit id field will kick in when simply adding the numeric field mapping.

IMO the inheritance relationship should be removed, allowing only the actual numeric field settings to be configured through numericField, whereas everything else such as store() would have to be configured through field() and thus naturally not be accessible through documentId().numericField().