len([]string) == 0 && err == nil => all namespaces.

CostFactorOfInAppFiltering = 4 means, we assume that the cost of listing all resources and filtering them in the app (instead of using kube-apiserver) is 5x of that via label based selection.

ref: https://github.com/coreos/prometheus-operator/blob/cc584ecfa08d2eb95ba9401f116e3a20bf71be8b/pkg/apis/monitoring/v1/types.go#L578 NamespaceSelector is a selector for selecting either all namespaces or a list of namespaces.

Reconciler reconciles a Release object.

ResourceRef contains information that points to the resource being used.