Running the density tool will report Pss and Rss total and per container
values for shim memory usage. Values are reported in KB.
```bash
containerd-stress density --count 500
INFO[0000] pulling docker.io/library/alpine:latest
INFO[0000] generating spec from image
{"pss":421188,"rss":2439688,"pssPerContainer":842,"rssPerContainer":4879}
```
Signed-off-by: Michael Crosby <crosbymichael@gmail.com>
