Set a longer metric retention time for Prometheus

roles/monitor/defaults/main.yml

@@ -3,3 +3,5 @@
 graphite_secret_key: "changeme"
 grafana_secret_key: "changeme"
 test_source_stream: "/test.ogg"
+prometheus_tsdb_retention_time: "180d"
+

roles/monitor/templates/prometheus.default.j2

-ARGS="--web.external-url=https://monitor.{{ public_domain }}"
+ARGS="--web.external-url=https://monitor.{{ public_domain }} --storage.tsdb.retention.time={{ prometheus_tsdb_retention_time }}"