blkio metrics are reported as zero

Likely due a change in semantics, worth making it work on a 5+ kernel though

# curl -s localhost:3909/metrics | grep -i blkio
# HELP cgroup_blkio_bytes Bytes read/written by blkio.
# TYPE cgroup_blkio_bytes counter
cgroup_blkio_bytes{mode="read",service="aa-create-maildirs.service",slice="system.slice"} 0
cgroup_blkio_bytes{mode="read",service="accountserver.service",slice="system.slice"} 0
cgroup_blkio_bytes{mode="read",service="auditd.service",slice="system.slice"} 0