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/tasks/main.yml

@@ -5,3 +5,7 @@
   ansible.builtin.include_tasks: pools.yml
 - name: "Setup ZFS datasets: filesystems, snapshots, volumes"
   ansible.builtin.include_tasks: datasets.yml
+
+- name: Setup ZFS capacity metrics
+  ansible.builtin.include_tasks: metrics.yml
+  when: zfs_metrics_enabled | bool