Reduce Docker CPU metrics
Reduce Docker CPU metrics

- `usage_system` was removed because it duplicates data already collected by
  CPU input
- `usage_total`, `usage_in_kernelmode` and `usage_in_usermode` were removed
  because `usage_percent` is a more useful representation of these values.
  Additionally, the unit of these metrics is difficult to determine and weird.

Instrumental/instrumentald,Instrumental/instrumentald,Instrumental/instrumentald