Add Root delay/dispersion metrics

[hhoffstaette]

This adds two additional metrics: Root delay & -dispersion. I hope I got the abbreviated descriptions right; they were taken from the man page.

[SuperQ]

I wonder if we should document the clock error calculation as a recording rule suggestion in the README.

groups:
  - name: Chrony
    rules:
      - record: instance:chrony_clock_error_seconds:abs
        expr: >
          abs(chrony_tracking_last_offset_seconds)
          +
          chrony_tracking_root_dispersion_seconds
          +
          (0.5 * chrony_tracking_root_delay_seconds)

[hhoffstaette]

I wonder if we should document the clock error calculation as a recording rule suggestion in the README.

I'm afraid I don't really know what a recording rule is :thinking: