prometheus: allow setting custom retention time

15 days is default and also too short.

roles/prometheus/README.md

@@ -1,3 +1,3 @@
 Install and configure prometheus.
 
-Job definitions should be placed in /etc/prometheus/conf.d by roles using this one.
+Job definitions should be placed in /etc/prometheus/conf.d by roles using this one. Data retention time can be set with the `retention` property in a NetBox config context.

roles/prometheus/handlers/main.yml

@@ -3,3 +3,9 @@
     name: prometheus
     state: reloaded
   when: "'handler' not in ansible_skip_tags"
+
+- name: restart prometheus
+  service:
+    name: prometheus
+    state: restarted
+  when: "'handler' not in ansible_skip_tags"

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/