Improve information about metrics

[katarzyna-z]

• Corrected information about perf metrics
• Add information about build flags required for metrics

[katarzyna-z]

/retest

[dashpole]

Looks like the divider is missing between container_perf_events_total and container_perf_metric_scaling_ratio, and the divider above libpfm for container_perf_events_total.

[katarzyna-z]

I don't see that... I see 5 dividers in lines with container_perf_events_total and 5 dividers in line with container_perf_metric_scaling_ratio

[dashpole]

I only see it when displaying the rich diff... not actually sure why it happens. We can fix it in a follow-up if needed.

[dashpole]

lgtm

[dashpole]

huh, I don't see the problem anymore in the actual page: https://github.com/google/cadvisor/blob/master/docs/storage/prometheus.md