grafana: replace disk-usage-collector with prometheus-zfs-exporter

The custom disk-usage-collector shell script + minutely timer is replaced
by prometheus-zfs-exporter (pdf/zfs_exporter, packaged in nixpkgs as
services.prometheus.exporters.zfs). The exporter provides pool capacity
metrics (allocated/free/size) natively.

Partition metrics (/boot, /persistent, /nix) now use node_exporter's
built-in filesystem collector (node_filesystem_*_bytes) which already
runs and collects these metrics.

Also fixes a latent race condition in serviceMountWithZpool: the -mounts
service now orders after zfs-mount.service (which runs 'zfs mount -a'),
not just after pool import. Without this, the mount check could run
before datasets are actually mounted.

service-configs.nix

@@ -189,6 +189,10 @@ rec {
         port = 9563;
         proto = "tcp";
       };
+      prometheus_zfs = {
+        port = 9134;
+        proto = "tcp";
+      };
       harmonia = {
         port = 5500;
         proto = "tcp";