Add Alertmanger metric target(s)

This commit enables scraping of Alertmanager metrics.

Change-Id: I69f4ac7de0f95eff393d9658af396e3c04824c8f

ansible/roles/prometheus/templates/prometheus.yml.j2

@@ -128,6 +128,13 @@ scrape_configs:
 {% endif %}
 
 {% if enable_prometheus_alertmanager | bool %}
+  - job_name: alertmanager
+    static_configs:
+      - targets:
+{% for host in groups['prometheus-alertmanager'] %}
+        - '{{ 'api' | kolla_address(host) | put_address_in_context('url') }}:{{ hostvars[host]['prometheus_alertmanager_port'] }}'
+{% endfor %}
+
 alerting:
   alertmanagers:
   - static_configs:

releasenotes/notes/add-alertmanager-metrics-a8d87b0793f2255a.yaml

@@ -0,0 +1,4 @@
+---
+features:
+  - |
+    Add support for Alertmanager metrics scraping in Prometheus.