feat(zfs): add capacity metrics and Grafana dashboard

Deploy an hourly systemd timer that writes pool and dataset capacity to
node_exporter's textfile collector, since --collector.zfs exposes only
ARC/I-O/state. Add a provisioned 'ZFS - Pools & Datasets' dashboard with
per-pool totals, per-dataset used/quota and daily growth panels.

roles/zfs/templates/zfs-metrics.timer.j2

@@ -0,0 +1,10 @@
+[Unit]
+Description=Timer for ZFS capacity metrics collection
+
+[Timer]
+OnCalendar={{ zfs_metrics_interval }}
+Persistent=true
+Unit=zfs-metrics.service
+
+[Install]
+WantedBy=timers.target