fix(monitoring): node-exporter extraArgs REPLACES the chart default, so restate it

Verified against the chart (helm show values prometheus-community/prometheus-node-exporter)
and against the live DaemonSet's argv on prod: the subchart's extraArgs default is
NOT empty — it ships the two filesystem excludes
(mount-points-exclude for /dev,/proc,/sys,containerd,docker,kubelet and the
fs-types-exclude list). Setting extraArgs to just --collector.mountstats would have
silently dropped both, so the filesystem collector would have begun reporting every
container overlay and every kubelet bind mount: a large cardinality jump, and a
behaviour change in NodeDiskFillingUp, which alerts per mountpoint. Both defaults
are now restated verbatim alongside the new collector, with a note to re-reconcile
them on a chart upgrade.

Co-Authored-By: Claude Opus 5 <[email protected]>

platform/monitoring/kube-prometheus-stack.yaml

@@ -396,7 +396,19 @@ spec:
       # same lie as an alert rule on a metric nobody scrapes. No alert rule references
       # node_mountstats_* yet, deliberately, for exactly that reason; the per-export latency rule
       # is the follow-up once the series are confirmed, and runbooks/node-io.md records the gap.
+      #
+      # extraArgs REPLACES, IT DOES NOT APPEND — and the chart's DEFAULT for this key is not empty.
+      # prometheus-node-exporter's own values.yaml ships two filesystem excludes in `extraArgs`
+      # (confirmed against the chart, and against the live DaemonSet's argv on prod). Listing only
+      # `--collector.mountstats` here would silently DROP both, so the filesystem collector would
+      # start reporting every container overlay and every kubelet bind mount: a large cardinality
+      # jump and a behaviour change in NodeDiskFillingUp, which alerts per mountpoint. Both
+      # defaults are therefore restated verbatim below. If the chart's defaults change on an
+      # upgrade, this list must be re-reconciled against
+      #   helm show values prometheus-community/prometheus-node-exporter | grep -A3 '^extraArgs:'
       extraArgs:
+        - --collector.filesystem.mount-points-exclude=^/(dev|proc|sys|run/containerd/.+|var/lib/docker/.+|var/lib/kubelet/.+)($|/)
+        - --collector.filesystem.fs-types-exclude=^(autofs|binfmt_misc|bpf|cgroup2?|configfs|debugfs|devpts|devtmpfs|fusectl|hugetlbfs|iso9660|mqueue|nsfs|overlay|proc|procfs|pstore|rpc_pipefs|securityfs|selinuxfs|squashfs|sysfs|tracefs|erofs)$
         - --collector.mountstats
       resources:
         requests: