Add node_name label to metrics

[lambdanis]

Currently metrics exposed by Tetragon don't have a label with the node name. node label is added by a relabeling in the default ServiceMonitor. This works well enough, however:

• In deployments not using the default ServiceMonitor, users have to manually configure scrape config to add such relabeling.
• In some deployments (non-k8s in particular) node name might be not available to Prometheus at the scrape time.
• node label on metrics might end up being different than node_name field in JSON events.
• It's always nice to have metrics labels matching events fields.

So I'm suggesting to add node_name label to all metrics exposed by Tetragon - it should be retrieved in the same way as node_name in events.