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.

services/grafana/prometheus.nix

@@ -44,6 +44,12 @@ in
         listenAddress = "127.0.0.1";
         apcupsdAddress = "127.0.0.1:3551";
       };
+
+      zfs = {
+        enable = true;
+        port = service_configs.ports.private.prometheus_zfs.port;
+        listenAddress = "127.0.0.1";
+      };
     };
 
     scrapeConfigs = [
@@ -89,6 +95,12 @@ in
           { targets = [ "127.0.0.1:${toString service_configs.ports.private.igpu_exporter.port}" ]; }
         ];
       }
+      {
+        job_name = "zfs";
+        static_configs = [
+          { targets = [ "127.0.0.1:${toString service_configs.ports.private.prometheus_zfs.port}" ]; }
+        ];
+      }
     ];
   };