metrics: density: measure runc containers
metrics: density: measure runc containers

This metrics test just allow measure the memory density
about Clear Containers, it fails when tries to measure
PSS memory of components that only exists during a Clear
container execution (e.g. qemu instance).

This commit allows to measure the PSS of the workload launched
by a runc container. Basically this commit includes a function
that will get all instances of docker-containerd-shim launched
by docker-containerd, then it will calculate the PSS average
about the child process launched by each docker-containerd-shim.

docker-containerd
                ├── docker-containerd-shim
                                          └── workload <--1
                ├── docker-containerd-shim
                                          └── workload <--2
                ├── docker-containerd-shim
                                          └── workload <--N
Fixes: #480

Signed-off-by: Mario Alfredo Carrillo Arevalo <962470f5d16f97233f38b7e7baf23f990a7fbec3@intel.com>

MarioCarrilloA/tests,chavafg/tests,chavafg/tests,clearcontainers/tests,MarioCarrilloA/tests,devimc/tests,clearcontainers/tests,devimc/tests