Add following metrics:
- used_inodes
- total_inodes
- healing
- online
- reads_per_sec
- reads_kb_per_sec
- reads_await
- writes_per_sec
- writes_kb_per_sec
- writes_await
- perc_util
To be able to calculate the `per_sec` values, we capture the IOStats-related
data in the beginning (along with the time at which they were captured),
and compare them against the current values subsequently. This is because
dividing by "time since server uptime." doesn't work in k8s environments.
08d3d06a06