runtimeVM: Calculate the WorkingSetBytes stats

WorkingSetBytes is the bit that needs to be set in order to provide
kubelet the pod's memory information.

Although it's calculated in a slightly different way for "oci" runtime
types, the logic is quite similar for the "vm" runtime type, with the
only difference being where the TotalInactiveFile information comes
from.

This is the last bit needed in order to have `kubectl top pod $pod`
working, as shown below:
```
[fidencio@localhost cri-o]$ kubectl get pods
NAME             READY   STATUS    RESTARTS   AGE
example-fedora   1/1     Running   0          130m

[fidencio@localhost cri-o]$ kubectl get pod example-fedora -o yaml | grep runtimeClassName
      {"apiVersion":"v1","kind":"Pod","metadata":{"annotations":{},"labels":{"app":"example-fedora-app"},"name":"example-fedora","namespace":"default"},"spec":{"containers":[{"args":["-m","http.server","8080"],"command":["python3"],"image":"fedora:33","name":"example-fedora","ports":[{"containerPort":8080}]}],"runtimeClassName":"kata"}}
        f:runtimeClassName: {}
  runtimeClassName: kata

[fidencio@localhost cri-o]$ kubectl top pod
NAME             CPU(cores)   MEMORY(bytes)
example-fedora   1m           9Mi
```

Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>

internal/oci/runtime_vm.go

@@ -737,14 +737,15 @@ func (r *runtimeVM) ContainerStats(c *Container, _ string) (*ContainerStats, err
 
 func metricsToCtrStats(c *Container, m *cgroups.Metrics) *ContainerStats {
 	var (
-		cpu         float64
-		cpuNano     uint64
-		memUsage    uint64
-		memLimit    uint64
-		memPerc     float64
-		blockInput  uint64
-		blockOutput uint64
-		pids        uint64
+		cpu             float64
+		cpuNano         uint64
+		memUsage        uint64
+		memLimit        uint64
+		memPerc         float64
+		workingSetBytes uint64
+		blockInput      uint64
+		blockOutput     uint64
+		pids            uint64
 	)
 
 	if m != nil {
@@ -756,6 +757,14 @@ func metricsToCtrStats(c *Container, m *cgroups.Metrics) *ContainerStats {
 		memUsage = m.Memory.Usage.Usage
 		memLimit = getMemLimit(m.Memory.Usage.Limit)
 		memPerc = float64(memUsage) / float64(memLimit)
+		if memUsage > m.Memory.TotalInactiveFile {
+			workingSetBytes = memUsage - m.Memory.TotalInactiveFile
+		} else {
+			logrus.Debugf(
+				"unable to account working set stats: total_inactive_file (%d) > memory usage (%d)",
+				m.Memory.TotalInactiveFile, memUsage,
+			)
+		}
 
 		for _, entry := range m.Blkio.IoServiceBytesRecursive {
 			switch strings.ToLower(entry.Op) {
@@ -768,16 +777,17 @@ func metricsToCtrStats(c *Container, m *cgroups.Metrics) *ContainerStats {
 	}
 
 	return &ContainerStats{
-		Container:   c.ID(),
-		CPU:         cpu,
-		CPUNano:     cpuNano,
-		SystemNano:  time.Now().UnixNano(),
-		MemUsage:    memUsage,
-		MemLimit:    memLimit,
-		MemPerc:     memPerc,
-		BlockInput:  blockInput,
-		BlockOutput: blockOutput,
-		PIDs:        pids,
+		Container:       c.ID(),
+		CPU:             cpu,
+		CPUNano:         cpuNano,
+		SystemNano:      time.Now().UnixNano(),
+		MemUsage:        memUsage,
+		MemLimit:        memLimit,
+		MemPerc:         memPerc,
+		WorkingSetBytes: workingSetBytes,
+		BlockInput:      blockInput,
+		BlockOutput:     blockOutput,
+		PIDs:            pids,
 	}
 }