Support disabling Prometheus server

This is useful to people who manage their Prometheus Server
externally to Kolla Ansible, or want to use the exporters with
another framework such as Monasca.

Change-Id: Ie3f61e2e186c8e77e21a7b53d2bd7d2a27eee18e

ansible/group_vars/all.yml

@@ -1013,6 +1013,7 @@ use_common_mariadb_user: "no"
 ############
 # Prometheus
 ############
+enable_prometheus_server: "{{ enable_prometheus | bool }}"
 enable_prometheus_haproxy_exporter: "{{ enable_haproxy | bool }}"
 enable_prometheus_mysqld_exporter: "{{ enable_mariadb | bool }}"
 enable_prometheus_node_exporter: "{{ enable_prometheus | bool }}"

ansible/roles/prometheus/defaults/main.yml

@@ -5,13 +5,13 @@ prometheus_services:
   prometheus-server:
     container_name: prometheus_server
     group: prometheus
-    enabled: true
+    enabled: "{{ enable_prometheus_server }}"
     image: "{{ prometheus_server_image_full }}"
     volumes: "{{ prometheus_server_default_volumes + prometheus_server_extra_volumes }}"
     dimensions: "{{ prometheus_server_dimensions }}"
     haproxy:
       prometheus_server:
-        enabled: "{{ enable_prometheus }}"
+        enabled: "{{ enable_prometheus_server }}"
         mode: "http"
         external: false
         port: "{{ prometheus_port }}"

ansible/roles/prometheus/tasks/precheck.yml

@@ -25,6 +25,7 @@
   when:
     - container_facts['prometheus_server'] is not defined
     - inventory_hostname in groups['prometheus']
+    - enable_prometheus_server | bool
 
 - name: Checking free port for Prometheus node_exporter
   wait_for:

etc/kolla/globals.yml

@@ -632,6 +632,7 @@
 ############
 # Prometheus
 ############
+#enable_prometheus_server: "{{ enable_prometheus | bool }}"
 #enable_prometheus_haproxy_exporter: "{{ enable_haproxy | bool }}"
 #enable_prometheus_mysqld_exporter: "{{ enable_mariadb | bool }}"
 #enable_prometheus_node_exporter: "{{ enable_prometheus | bool }}"

releasenotes/notes/support-disabling-prometheus-server-907be6846bc364df.yaml

@@ -0,0 +1,6 @@
+---
+features:
+  - |
+    Prometheus server can now be disabled, allowing the exporters to be
+    deployed without it. The default behaviour of deploying Prometheus
+    server when Prometheus is enabled remains.