OVN-K alerts: add northd inactivity

We want to detect when an northd instance is not active. Northd is within a HA cluster. We can detect the status of this HA cluster with metric:

ovn_northd_status gauge

• Description
• Specifies whether this instance of ovn-northd is standby(0) or active(1) or paused(2)

• Normal/expected values
• 0 or 1

PR that includes the commit for this change: https://github.com/openshift/cluster-network-operator/pull/1526

See commit titled: OVN-K alerts: add ovn northd inactivity alert

See commit code for alert OVNKubernetesNorthdInactive.