fix: Incorrect values for cluster var

[ajax-kovtunov-o]

The var cluster collects values from the cnpg_collector_up metric by name cluster. I see that cluster is a Kubernetes cluster name, then exported_cluster is a real PostgreSQL cluster name. So, without adjustments grafana dashboard won't work.

cnpg_collector_up{
cluster="cluster_name",
container="postgres",
endpoint="metrics",
environemnt="prod",
exported_cluster="grafana-db-cluster",
instance="hidden_ip",
job="monitoring/grafana-db-cluster",
namespace="monitoring",
pod="grafana-db-cluster-1",
prometheus="monitoring/victoria-metrics-k8s-stack-vm"
}

[ajax-kovtunov-o]

@gbartolini pls look

[ajax-kovtunov-o]

@mnencia