prometheus: allow setting custom retention time

15 days is default and also too short.

roles/prometheus/tasks/main.yml

@@ -8,6 +8,14 @@
     path: /etc/prometheus/conf.d
     state: directory
 
+- name: Configure prometheus retention time
+  when: retention is defined
+  lineinfile:
+    path: /etc/conf.d/prometheus
+    regexp: "^prometheus_retention_time="
+    line: "prometheus_retention_time={{ retention }}"
+  notify: restart prometheus
+
 - name: Configure prometheus
   copy:
     dest: /etc/prometheus/