Proposal: Add metrics for function resource usage

[Qolzam]

Add metrics for resource usage of each function in OFC dashboard.

1. Memory usage
2. CPU usage

Expected Behaviour

Display resource usage for function in OFC dashboard.

Current Behaviour

Now OFC dashboard only show the number of function invocation in code status (success, error).

Possible Solution

We can have some metrics like container_cpu_usage_seconds_total from Prometheus.

[mr47]

@Qolzam You can use dockprom/swamprom for getting these statistics. In any case, I think OpenFaaS can't store cpu/memory without cadvisor and /maybe node-exporter. So I think it's more about to how you will configure it. Checkout dockprom/swamprom.