Cgroup monitoring and alerting

[pingandb]

Feature Request

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

There isn't yet a mechanism to monitor the cgroup cpu/memory limit and do corresponding alerting.

Describe the feature you'd like:

Add a monitor to watch cgroup cpu/memory limit setting changes and update corresponding metrics. Display the limits on related Grafana graphs. Trigger alerting when cpu/memory usage exceeds a threshold relative to the limit.

Describe alternatives you've considered:

None.

Teachability, Documentation, Adoption, Migration Strategy:

In real scenarios, users like to change cpu/memory limits online w/o restarting the service, show the current limit somewhere and receive alerting when the usage is approaching the limit.