Provide custom metrics.

[Gerrit91]

❯ curl -sSq localhost:2112/metrics | grep firewall_deployment 
# HELP firewall_deployment_ready_replicas provide information on firewall deployment ready replicas
# TYPE firewall_deployment_ready_replicas gauge
firewall_deployment_ready_replicas{name="shoot-firewall",namespace="shoot--pbs4kr--inttest10"} 1
# HELP firewall_deployment_target_replicas provide information on firewall deployment target replicas
# TYPE firewall_deployment_target_replicas gauge
firewall_deployment_target_replicas{name="shoot-firewall",namespace="shoot--pbs4kr--inttest10"} 1