Delete metrics with policy label on policy delete

[lambdanis]

tetragon_policy_events_total metric has a policy label. When a TracingPolicy is deleted, the corresponding metrics are still exposed by Tetragon, leading to an overhead (usually not big, as policies are not churned frequently, but might be significant in some cases).

There was a similar problem with metrics that have a pod label. Now when a pod is deleted, Tetragon deletes its metrics from the registry (see https://github.com/cilium/tetragon/blob/main/pkg/metrics/metricwithpod.go).

We should add a similar handler for policy delete events. One small gotcha is that at the moment metrics don't distinguish between clusterwide and namespaced policies, so we should first add another label to distinguish between different policy kinds.