Instance stats CPU usage

[dtrudg]

Is your feature request related to a problem? Please describe.

CPU usage of an instance should be shown in a percentage value, but is currently in nanoseconds.

Describe the solution you'd like

CPU usage is shown as a percentage, and updates continuously for each interval when #892 interactive monitoring is implemented.

Additional context

CPU usage percentage must be calculated over time, between two measurement points:

• Obtain the cpu usage in nanoseconds
• After a wait of 1 second (or other fixed duration), obtain the cpu usage in nanoseconds again.
• Compute CPU usage as the difference in nanoseconds used, divided by the total CPU nanoseconds available (the known duration)

This means that even in non-interactive mode, the instance stats command must take two observations of the resource usage of the container.