pkg/metrics: add metrics for policy kernel memory use

Similarly to TracingPolicy state, you can retrieve this information
through the gRPC API by listing the policies or using the metrics
interface.

Signed-off-by: Mahe Tardy <mahe.tardy@gmail.com>

pkg/metrics/policymetrics/policymetrics.go

@@ -32,12 +32,19 @@ var policyState = metrics.MustNewCustomGauge(metrics.NewOpts(
 	nil, []metrics.ConstrainedLabel{stateLabel}, nil,
 ))
 
+var policyKernelMemory = metrics.MustNewCustomGauge(metrics.NewOpts(
+	consts.MetricsNamespace, "", "tracingpolicy_kernel_memory_bytes",
+	"The amount of kernel memory in bytes used by policy's sensors non-shared BPF maps (memlock).",
+	nil, nil, []metrics.UnconstrainedLabel{{Name: "policy", ExampleValue: "example-policy"}},
+))
+
 // This metric collector converts the output of ListTracingPolicies into a few
 // gauges metrics on collection. Thus, it needs a sensor manager to query.
 func NewPolicyCollector() metrics.CollectorWithInit {
 	return metrics.NewCustomCollector(
 		metrics.CustomMetrics{
 			policyState,
+			policyKernelMemory,
 		},
 		collect,
 		collectForDocs,
@@ -63,6 +70,7 @@ func collect(ch chan<- prometheus.Metric) {
 	for _, policy := range list.Policies {
 		state := policy.State
 		counters[state]++
+		ch <- policyKernelMemory.MustMetric(float64(policy.KernelMemoryBytes), policy.Name)
 	}
 
 	ch <- policyState.MustMetric(
@@ -87,4 +95,5 @@ func collectForDocs(ch chan<- prometheus.Metric) {
 	for _, state := range stateLabel.Values {
 		ch <- policyState.MustMetric(0, state)
 	}
+	ch <- policyKernelMemory.MustMetric(0, "example-policy")
 }