in_cpu: Collect iowait, irq, softirq and steal

**Is your feature request related to a problem? Please describe.**
fluent-bit currently only collects/calculates CPU percentages for user+nice and system. It ignores the values for iowait, irq, softirq and steal.

**Describe the solution you'd like**
In addition to collecting/calculating CPU usage for user and sys, it would be great if iowait, irq, softirq and steal were collected as well. This would probably also provide a more accurate total figure.

**Additional context**
Depending on the workload iowait and irq/softirq values may be significant and in cloud environment being able to track CPU steal is often very useful.