Allow encoding all unknown cgroups as a single item

[bobrik]

Currently short lived cgroups can create many items:

ebpf_exporter_cgroup_rstat_flush_seconds_sum{cgroup="unknown_cgroup_id:2196562",level="2"} 224
ebpf_exporter_cgroup_rstat_flush_seconds_sum{cgroup="unknown_cgroup_id:2196598",level="2"} 305
ebpf_exporter_cgroup_rstat_flush_seconds_sum{cgroup="unknown_cgroup_id:2196670",level="2"} 172
ebpf_exporter_cgroup_rstat_flush_seconds_sum{cgroup="unknown_cgroup_id:2196706",level="2"} 167
ebpf_exporter_cgroup_rstat_flush_seconds_sum{cgroup="unknown_cgroup_id:2196742",level="2"} 210

It might be good to allow these to be collapsed into one uknown_cgroup to keep cardinality in check.