bugfix: refactor alerts to accomodate for single-node clusters

For the sake of brevity, let:
Q:  kube_node_status_allocatable{job="kube-state-metrics",resource="cpu"} (allocable), and,
QQ: namespace_cpu:kube_pod_container_resource_requests:sum{} (requested), thus,
most quota commits relevant here exist in the form: sum(QQ) by (cluster) - (sum(Q) by (cluster) - max(Q) by (cluster)) > 0 and (sum(Q) by (cluster) - max(Q) by (cluster)) > 0,
which, in case of a single-node cluster (sum(Q) by (cluster) = max(Q) by (cluster)), is reduced to,
sum(QQ) by (cluster) > 0, i.e., the alert will fire if *any* request limits exist.

To address this, drop the "max(Q) by (cluster)" buffer assumed in
non-SNO clusters from SNO, reducing the expression to: sum(QQ) by (cluster) - sum(Q) by (cluster) > 0 (total requeted - total allocable > 0 to trigger alert),
since there is only a single node, so a buffer of the same sort does not
make sense.

alerts/resource_alerts.libsonnet

@@ -34,18 +34,34 @@
           } +
           if $._config.showMultiCluster then {
             expr: |||
-              sum(namespace_cpu:kube_pod_container_resource_requests:sum{%(ignoringOverprovisionedWorkloadSelector)s}) by (%(clusterLabel)s) - (sum(kube_node_status_allocatable{%(kubeStateMetricsSelector)s,resource="cpu"}) by (%(clusterLabel)s) - max(kube_node_status_allocatable{%(kubeStateMetricsSelector)s,resource="cpu"}) by (%(clusterLabel)s)) > 0
+              (count(kube_node_info) == 1
               and
-              (sum(kube_node_status_allocatable{%(kubeStateMetricsSelector)s,resource="cpu"}) by (%(clusterLabel)s) - max(kube_node_status_allocatable{%(kubeStateMetricsSelector)s,resource="cpu"}) by (%(clusterLabel)s)) > 0
+              sum(namespace_cpu:kube_pod_container_resource_requests:sum{%(ignoringOverprovisionedWorkloadSelector)s}) by (%(clusterLabel)s) -
+              sum(kube_node_status_allocatable{%(kubeStateMetricsSelector)s,resource="cpu"}) by (%(clusterLabel)s) > 0)
+              or
+              (sum(namespace_cpu:kube_pod_container_resource_requests:sum{%(ignoringOverprovisionedWorkloadSelector)s}) by (%(clusterLabel)s) -
+              (sum(kube_node_status_allocatable{%(kubeStateMetricsSelector)s,resource="cpu"}) by (%(clusterLabel)s) -
+              max(kube_node_status_allocatable{%(kubeStateMetricsSelector)s,resource="cpu"}) by (%(clusterLabel)s)) > 0
+              and
+              (sum(kube_node_status_allocatable{%(kubeStateMetricsSelector)s,resource="cpu"}) by (%(clusterLabel)s) -
+              max(kube_node_status_allocatable{%(kubeStateMetricsSelector)s,resource="cpu"}) by (%(clusterLabel)s)) > 0)
             ||| % $._config,
             annotations+: {
               description: 'Cluster {{ $labels.%(clusterLabel)s }} has overcommitted CPU resource requests for Pods by {{ $value }} CPU shares and cannot tolerate node failure.' % $._config,
             },
           } else {
             expr: |||
-              sum(namespace_cpu:kube_pod_container_resource_requests:sum{%(ignoringOverprovisionedWorkloadSelector)s}) - (sum(kube_node_status_allocatable{resource="cpu", %(kubeStateMetricsSelector)s}) - max(kube_node_status_allocatable{resource="cpu", %(kubeStateMetricsSelector)s})) > 0
+              (count(kube_node_info) == 1
+              and
+              sum(namespace_cpu:kube_pod_container_resource_requests:sum{%(ignoringOverprovisionedWorkloadSelector)s}) -
+              sum(kube_node_status_allocatable{resource="cpu", %(kubeStateMetricsSelector)s}) > 0)
+              or
+              (sum(namespace_cpu:kube_pod_container_resource_requests:sum{%(ignoringOverprovisionedWorkloadSelector)s}) -
+              (sum(kube_node_status_allocatable{resource="cpu", %(kubeStateMetricsSelector)s}) -
+              max(kube_node_status_allocatable{resource="cpu", %(kubeStateMetricsSelector)s})) > 0
               and
-              (sum(kube_node_status_allocatable{resource="cpu", %(kubeStateMetricsSelector)s}) - max(kube_node_status_allocatable{resource="cpu", %(kubeStateMetricsSelector)s})) > 0
+              (sum(kube_node_status_allocatable{resource="cpu", %(kubeStateMetricsSelector)s}) -
+              max(kube_node_status_allocatable{resource="cpu", %(kubeStateMetricsSelector)s})) > 0)
             ||| % $._config,
             annotations+: {
               description: 'Cluster has overcommitted CPU resource requests for Pods by {{ $value }} CPU shares and cannot tolerate node failure.' % $._config,