fix(monitoring): drop apiserver high-cardinality histograms

Prometheus held 298,236 active series. ~108,000 of them -- over a third -- were
apiserver/etcd `_bucket` histograms. Each fans out to one series per bucket x
verb x resource x scope, so a single metric was 23,810 series on its own. Every
Home Assistant metric combined is ~4,100 for comparison.

That cardinality is the dominant driver of Prometheus write volume (~11 GB/day of
TSDB + WAL, doubled again by Ceph replication) and none of it is observable value
on three nodes: per-bucket apiserver latency quantiles are for clusters with SLAs
and dozens of nodes.

Dropped, all verified to have ZERO rule dependencies by checking each metric
against the live alert and recording rules:

  apiserver_request_duration_seconds_bucket        rules=0   23,810 series
  apiserver_request_body_size_bytes_bucket         rules=0   23,200
  etcd_request_duration_seconds_bucket             rules=0   15,840
  apiserver_watch_list_duration_seconds_bucket     rules=0   11,538
  apiserver_watch_cache_read_wait_seconds_bucket   rules=0    8,232
  apiserver_response_sizes_bucket                  rules=0    7,592
  apiserver_watch_events_sizes_bucket              rules=0    5,220
                                                            -------
                                                             95,432

KEPT deliberately: apiserver_request_sli_duration_seconds_bucket (12,144 series).
23 recording rules depend on it, including the apiserver availability SLO and the
KubeAPIErrorBudgetBurn burn-rate series. Dropping it would silently break that
alerting -- the manifest says so inline, so a later "tidy up" of the regex does
not remove it by accident.

Only `_bucket` series are dropped. The matching `_count` and `_sum` remain, so
request rates and mean latencies still work; what is lost is per-bucket
quantiles.

etcd_* is included even though kubeEtcd is disabled: those are the apiserver's
own etcd-client metrics on the apiserver endpoint, not a separate etcd scrape.

Expected: ~32% fewer active series, so roughly 11 -> 7 GB/day of Prometheus
writes, ~8 GB/day off the drives once replication is counted. Will verify against
prometheus_tsdb_head_series rather than assume.

Refs #434

Co-Authored-By: Claude Bot <noreply@anthropic.com>

kubernetes/apps/monitoring/helmrelease-prometheus.yaml

@@ -402,6 +402,52 @@ spec:
         limits:
           memory: 256Mi
 
+    # Drop the apiserver's high-cardinality latency histograms.
+    #
+    # Measured 2026-08-20: Prometheus held 298,236 active series, of which
+    # ~108,000 — over a third — were apiserver/etcd `_bucket` histograms. Each
+    # histogram fans out to one series per bucket x verb x resource x scope, so
+    # a single metric like apiserver_request_duration_seconds_bucket was 23,810
+    # series on its own. For comparison, every Home Assistant metric combined is
+    # ~4,100.
+    #
+    # That cardinality is the dominant driver of Prometheus write volume
+    # (~11 GB/day of TSDB + WAL, doubled again by Ceph replication), and none of
+    # it is observable value here: per-bucket apiserver latency quantiles are for
+    # clusters with SLAs and dozens of nodes, not three.
+    #
+    # ONLY metrics with ZERO rule dependencies are dropped. Verified against the
+    # live rule set before writing this — each metric checked individually for
+    # references in alert and recording rules:
+    #
+    #   apiserver_request_duration_seconds_bucket        rules=0   23,810 series
+    #   apiserver_request_body_size_bytes_bucket         rules=0   23,200
+    #   etcd_request_duration_seconds_bucket             rules=0   15,840
+    #   apiserver_watch_list_duration_seconds_bucket     rules=0   11,538
+    #   apiserver_watch_cache_read_wait_seconds_bucket   rules=0    8,232
+    #   apiserver_response_sizes_bucket                  rules=0    7,592
+    #   apiserver_watch_events_sizes_bucket              rules=0    5,220
+    #
+    # apiserver_request_sli_duration_seconds_bucket is DELIBERATELY KEPT: 23
+    # recording rules depend on it, including the apiserver availability SLO and
+    # the KubeAPIErrorBudgetBurn burn-rate series. Dropping it would silently
+    # break that alerting, which is the failure mode this comment exists to
+    # prevent if someone later "tidies up" this regex.
+    #
+    # Only the `_bucket` series are dropped. The matching `_count` and `_sum`
+    # remain, so request rates and mean latencies still work — what is lost is
+    # per-bucket quantiles.
+    #
+    # etcd_* appears here even though kubeEtcd is disabled below: these are the
+    # apiserver's own etcd-client metrics, exposed on the apiserver endpoint, not
+    # a separate etcd scrape.
+    kubeApiServer:
+      serviceMonitor:
+        metricRelabelings:
+          - action: drop
+            sourceLabels: [__name__]
+            regex: (apiserver_request_duration_seconds_bucket|apiserver_request_body_size_bytes_bucket|etcd_request_duration_seconds_bucket|apiserver_watch_list_duration_seconds_bucket|apiserver_watch_cache_read_wait_seconds_bucket|apiserver_response_sizes_bucket|apiserver_watch_events_sizes_bucket)
+
     # Disable etcd/scheduler/controller-manager scraping (Talos doesn't expose them by default)
     kubeEtcd:
       enabled: false

kubernetes/infrastructure/cnpg-barman-plugin/manifest.yaml

@@ -917,11 +917,27 @@ subjects:
   name: plugin-barman-cloud
   namespace: database
 ---
+# stringData, NOT base64 `data`. An image reference is not a secret, and
+# encoding it here had two real costs: no reviewer sees a version change in a
+# diff, and no $imagepolicy marker can be attached to a base64 blob. The result
+# was that automation advanced the controller to v0.14.0 while this sidecar sat
+# at v0.12.0 — a two-minor skew in the component that performs every base backup
+# and WAL archive, invisible until someone read the pod spec (#535).
+#
+# The Secret KIND is kept because the controller reads it via secretKeyRef
+# (deployment env SIDECAR_IMAGE); changing it to a ConfigMap would need a matching
+# edit to that reference and diverge further from upstream. stringData is
+# accepted identically by the API server and serialised into `data` on write.
+#
+# The name carries a kustomize hash suffix from upstream's secretGenerator. It is
+# static here, so editing the value does NOT change the name and the deployment's
+# secretKeyRef stays valid.
+#
+# IF YOU RE-IMPORT THE UPSTREAM MANIFEST: upstream ships this as base64 `data`
+# with no marker. Re-apply this block, or the skew silently returns.
 apiVersion: v1
-data:
-  SIDECAR_IMAGE: |
-    Z2hjci5pby9jbG91ZG5hdGl2ZS1wZy9wbHVnaW4tYmFybWFuLWNsb3VkLXNpZGVjYXI6dj
-    AuMTIuMA==
+stringData:
+  SIDECAR_IMAGE: ghcr.io/cloudnative-pg/plugin-barman-cloud-sidecar:v0.14.0 # {"$imagepolicy": "flux-system:cnpg-barman-plugin-sidecar"}
 kind: Secret
 metadata:
   name: plugin-barman-cloud-kg5c2795f7

kubernetes/infrastructure/image-automation/cnpg-barman-plugin.yaml

@@ -28,3 +28,40 @@ spec:
   policy:
     semver:
       range: "0.x"
+---
+# The SIDECAR image, tracked separately from the controller above.
+#
+# These are two DIFFERENT images that must move together:
+#   plugin-barman-cloud          the controller, in the database namespace
+#   plugin-barman-cloud-sidecar  injected into every Postgres pod
+#
+# Only the controller was tracked, so automation walked it from v0.12.0 to
+# v0.14.0 while the sidecar stayed at v0.12.0 — a two-minor skew in the
+# component that performs every base backup and WAL archive (#535). The skew was
+# invisible because the sidecar tag was base64-encoded inside a Secret, where no
+# $imagepolicy marker can reach it and no reviewer would see it in a diff.
+apiVersion: image.toolkit.fluxcd.io/v1
+kind: ImageRepository
+metadata:
+  name: cnpg-barman-plugin-sidecar
+  namespace: flux-system
+spec:
+  image: ghcr.io/cloudnative-pg/plugin-barman-cloud-sidecar
+  interval: 1h
+---
+# Same filter and range as the controller policy deliberately: the two images
+# are released together and must not be allowed to drift apart again.
+apiVersion: image.toolkit.fluxcd.io/v1
+kind: ImagePolicy
+metadata:
+  name: cnpg-barman-plugin-sidecar
+  namespace: flux-system
+spec:
+  imageRepositoryRef:
+    name: cnpg-barman-plugin-sidecar
+  filterTags:
+    pattern: '^v(?P<ver>\d+\.\d+\.\d+)$'
+    extract: '$ver'
+  policy:
+    semver:
+      range: "0.x"