Implements three metrics for nfd-gc:
- nfd_gc_build_info: version information of nfd-gc.
- nfd_gc_objects_deleted_total: total number of NodeFeature and
NodeResourceTopology objects deleted by nfd-gc.
- nfd_gc_object_delete_failures_total: number of errors encountered when
deleting NodeFeature and NodeResourceTopology objects.
Hook into the same logic already exercised for NodeResourceTopology
objects: GC watches for node delete events and immediately drops stale
objects (NRT and now also NF). In addition there is a periodic resync to
catch any missed node deletes, once every hour by default.
