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.
overrideDerivation has been deprecated since 2019. The new
overrideAttrs properly handles the env attribute set used by
modern derivations to avoid the NIX_CFLAGS_COMPILE overlap
error between env and top-level derivation arguments.
