jsonnet: Drop cAdvisor metrics with no (pod, namespace) labels while...

jsonnet: Drop cAdvisor metrics with no (pod, namespace) labels while preserving ability to monitor system services resource usage

The following provides a description and cardinality estimation based on the tests in a local cluster:

container_blkio_device_usage_total - useful for containers, but not for system services (nodes*disks*services*operations*2)
container_fs_.*                    - add filesystem read/write data (nodes*disks*services*4)
container_file_descriptors         - file descriptors limits and global numbers are exposed via (nodes*services)
container_threads_max              - max number of threads in cgroup. Usually for system services it is not limited (nodes*services)
container_threads                  - used threads in cgroup. Usually not important for system services (nodes*services)
container_sockets                  - used sockets in cgroup. Usually not important for system services (nodes*services)
container_start_time_seconds       - container start. Possibly not needed for system services (nodes*services)
container_last_seen                - Not needed as system services are always running (nodes*services)
container_spec_.*                  - Everything related to cgroup specification and thus static data (nodes*services*5)