Increase default Prometheus tsdb retention horizon

To 90 days by default (configurable with prometheus_tsdb_retention).

roles/prometheus/defaults/main.yml

@@ -10,3 +10,6 @@ alertmanager_smtp_require_tls: false
 alertmanager_smtp_hello: "localhost"
 alertmanager_smtp_auth_username: ""
 alertmanager_smtp_auth_password: ""
+
+# Keep 3 months of timeseries data by default.
+prometheus_tsdb_retention: "90d"

roles/prometheus/templates/prometheus.default.j2

-ARGS="--web.external-url=https://monitor.{{ domain_public[0] }}"
+ARGS="--storage.tsdb.retention={{ prometheus_tsdb_retention }} --web.external-url=https://monitor.{{ domain_public[0] }}"