Add alert rules when OpenStack services are down

The openstack-exporter generate metrics that checks if services
are up. E.g:
- openstack_loadbalancer_up
- openstack_designate_up
- openstack_identity_up

This new rule uses a regex to identify all metrics name that
starts with 'openstack' and ends with 'up'. Case the result is 0,
meaning service is down, it will generate individual alerts for
every service with problems.

Change-Id: Ia3f6aced5dcbfa124b4340ad054a43a460284019

charms/openstack-exporter-k8s/src/prometheus_alert_rules/general.yaml

@@ -0,0 +1,16 @@
+groups:
+- name: OpenStackServices
+  rules:
+    - alert: OpenStackServicesDown
+      expr: |
+        sum by(service) (
+          label_replace({__name__=~"openstack_(.+)_up"}, "service", "$1", "__name__", "openstack_(.+)_up")
+        ) == 0
+      for: 5m
+      labels:
+        severity: critical
+        service: "{{ $labels.service }}"
+      annotations:
+        summary: OpenStack Services Down
+        description: |
+          The OpenStack service {{ $labels.service }} is down