Decide what to do with UnparseableDescriptors while synchronizing from another instance

Let's find out, if a code change is needed or things are fine as they are.

Current situation when one CollecTor fetches data from another CollecTor: Metrics Lib cannot parse the fetched content, provides an object of UnparseableDescriptor, and CollecTor simply ignores this object as unknown (only logs on trace level).

During normal collecting an unparseable descriptor is stored anyway.