Fix not reporting memory usage

[spowelljr]

Fixes #64

When I was running crictl stats I wasn't getting any memory usage reported, everything was 0.

$ crictl stats
CONTAINER           CPU %               MEM                 DISK                INODES
151dfb4aa44a8       2.33                0B                  0B                  0
8396e703be70c       0.40                0B                  0B                  0
8d098a7107992       0.00                0B                  2.294kB             0
d1431067b2940       0.19                0B                  0B                  0
dbce1442651b2       6.69                0B                  0B                  0
f4c66fafc1f19       0.43                0B                  0B                  0
f8e33de11bb58       0.30                0B                  0B                  0
fe27b6c89bdb6       3.57                0B                  0B                  0

I checked the response of:

cri-dockerd/libdocker/kube_docker_client.go

Line 612 in 5d7c9ba

response, err := d.client.ContainerStats(ctx, id, false)

And it was:

{
   ...
   "memory_stats":{
      "usage":14479360,
      "stats":{...},
      "limit":6232580096
   },
   ...
}

There is no PrivateWorkingSet field being returned from Docker, changing it to Usage resulted in the following:

CONTAINER           CPU %               MEM                 DISK                INODES
219a899e5607d       0.31                11.55MB             0B                  0
24b4c111d2f5a       0.40                15.67MB             0B                  0
282e95aafc4b6       0.54                13.65MB             0B                  0
6751f1fd2b742       0.01                10.96MB             2.294kB             0
675fb173de807       2.75                43.97MB             0B                  0
8a3fa3d62f6e6       0.26                7.995MB             0B                  0
edc7e0037532c       7.35                345MB               0B                  0
ffadb0bd1808d       2.27                29.68MB             0B                  0

If you look at the source code of Docker you can see that Windows uses PrivateWorkingSet but Unix uses Usage.

Windows:
https://github.com/moby/moby/blob/548614694305d10e25665b4a7d0f61b025d47dd1/daemon/daemon_windows.go#L522

Unix:
https://github.com/moby/moby/blob/548614694305d10e25665b4a7d0f61b025d47dd1/daemon/daemon_unix.go#L1475