feat(obs): make oauth2-proxy auth failures visible — alert on the gap, not on absent metrics

Jackett failed on 2026-09-30 in the most expensive way available: HTTP 200 and a
plausible page. oauth2-proxy answers an unauthenticated XHR with a 302 to Keycloak
rather than a 401; the browser follows it silently against a live SSO session and
the call resolves 200 carrying a 44 KB HTML dashboard where the app's JavaScript
wanted JSON. The user is never re-prompted, reloading never helps, and every uptime
check stays green. `jackett-down` watches replicas_available, which was 1 throughout.

The reason nothing alerted turns out not to be a missing rule. Measured on prod:

    count by (__name__) ({__name__=~"oauth2_proxy.*"})
      oauth2_proxy_requests_in_flight => 2

Two series, both from the SHARED forward-auth Deployment. The six per-app gate
sidecars (crafty, jackett, pihole, qbittorrent, radarr, sonarr) publish nothing at
all: no --metrics-address, so the metrics listener never starts; no `metrics`
container port; no ServiceMonitor or PodMonitor selects them. The alert the incident
calls for — sustained auth redirects to an authenticated user — is therefore NOT
WRITABLE, and writing it anyway would have shipped a rule that is present,
evaluating, permanently silent and indistinguishable from health.

So alert on the condition that made the incident invisible, from families that ARE
scraped:

  OAuth2ProxyGateUnobserved
    count by (namespace) (kube_pod_container_info{container="oauth2-proxy"})
      unless
    count by (namespace) (up{container="oauth2-proxy"})

kube_pod_container_info is kube-state-metrics (a declared scrape owner) and sees the
container whether or not it exports anything; `up` is UNIVERSAL and carries a
`container` label for any target, so the rule needs no advance knowledge of what job
name a future per-app scrape picks. Nothing in it touches oauth2_proxy_*, so it
cannot be inert for the reason it is reporting. Run verbatim against live prod it
returns six namespaces and correctly excludes oauth2-proxy. It clears per namespace
as each gate gains a real scrape.

Also:
- promtool companion test pinning all four states: fires unscraped, quiet when
  scraped, honours for:1h, and STOPS once a scrape appears under a different job name.
- runbooks/oauth2-proxy-auth-failures.md — tell the gate's refusal from the group
  check from the app's own login from an XHR that silently got HTML, with the
  2026-09-30 jackett trace as the worked example and the Loki queries to use while
  there are no metrics. Registered in the configMapGenerator and README.
- runbooks/README.md gains an inert-alerts row for the unscraped sidecars.

Rejected: an alert on the shared proxy's oauth2_proxy_requests_total ratio. It needs
a conformance/ SCRAPE_OWNERS entry (human approval) and, decisively, would not have
fired on this incident — the shared instance is not in any sidecar-gated app's data
path and its current pods have proxied nothing. It would have looked like coverage
and been none.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

platform/oauth2-proxy/auth-visibility-alerts.yaml

@@ -0,0 +1,113 @@
+---
+# oauth2-proxy AUTH-VISIBILITY alerting (R-OBS-04).
+#
+# ============================================================================================
+# WHY THIS FILE EXISTS — THE 2026-09-30 JACKETT FAILURE THAT NOTHING OBSERVED
+# ============================================================================================
+# Jackett's UI stopped working. Reloading never fixed it and the user was never re-prompted to
+# log in. The sidecar's own access log had the whole story:
+#
+#   [oauthproxy.go:1026] No valid authentication in request. Initiating login.
+#   GET "/api/v2.0/indexers/yts/config?_=..."          HTTP/1.1 ... 302 400
+#   GET "/api/v2.0/indexers/thepiratebay/config?_=..." HTTP/1.1 ... 302 409
+#   ... ten of these at 15:30:53, then a fresh [AuthSuccess] at 15:31:06,
+#   then "No valid authentication in request" again 26 SECONDS LATER, inside the same session.
+#
+# NOTHING ANYWHERE ELSE RECORDED IT. oauth2-proxy answers an unauthenticated XHR with a 302 to
+# Keycloak rather than a 401; the browser follows it silently, the live SSO session satisfies it
+# without a prompt, and the XHR resolves 200 carrying a 44 KB HTML dashboard where the app's
+# JavaScript wanted JSON. From the browser, from the user, and from any uptime check, the service
+# is healthy. `jackett-down` (apps/wave-0/jackett.yaml) watches
+# kube_deployment_status_replicas_available, which was 1 throughout and would have been 1 if the
+# gate had rejected every request ever made.
+#
+# ============================================================================================
+# WHY THIS FILE DOES NOT CONTAIN THE OBVIOUS ALERT — MEASURED, prod, 2026-09-30
+# ============================================================================================
+# The alert you would want is "a gated app is serving a sustained rate of auth redirects to a
+# user who is already authenticated". It is NOT WRITABLE against this cluster's TSDB, and the
+# reason is this file's real subject:
+#
+#   THE PER-APP GATE SIDECARS EXPORT NO METRICS AT ALL.
+#
+#   $ promtool query instant :9090 'count by (__name__) ({__name__=~"oauth2_proxy.*"})'
+#     oauth2_proxy_requests_in_flight => 2          <- the SHARED proxy's two replicas. That is ALL.
+#
+#   Every sidecar (crafty, jackett, pihole, qbittorrent, radarr, sonarr) runs with
+#   `--http-address=0.0.0.0:4180` and NO `--metrics-address`, so oauth2-proxy's metrics listener
+#   is never started; the container declares only port `http:4180`, there is no `metrics` port on
+#   any of their Services, and there is no ServiceMonitor or PodMonitor for any of them anywhere
+#   in the cluster (measured: `kubectl get servicemonitors,podmonitors -A`). Six gates, zero
+#   telemetry. An alert written against `oauth2_proxy_requests_total{namespace="jackett"}` would
+#   have been PERMANENTLY INERT — present, evaluating, silent, and indistinguishable from health.
+#   This repo has shipped that mistake before (VeleroNoRecentBackup, the Longhorn rules), which is
+#   why `conformance/tests/l0-static/alert-metrics-have-scrape-targets` exists, and why the
+#   temptation was refused here rather than ratcheted into a register.
+#
+#   The SHARED proxy IS scraped (job="oauth2-proxy", 2 targets, up=1) but it is not in the data
+#   path for any sidecar-gated app — each app gates itself — and its current pods export no
+#   `oauth2_proxy_requests_total` at all, having proxied nothing since they started. An alert on
+#   the shared instance would not have fired on this incident either. Saying so is the point.
+#
+# So this file alerts on the CONDITION THAT MADE THE INCIDENT INVISIBLE, not on the incident.
+# That condition is measurable today, from two already-scraped metric families, and it is the
+# precondition for ever being able to alert on the incident itself. It self-clears the moment a
+# sidecar gains a metrics endpoint and a scrape — at which point the auth-churn rule becomes
+# writable and belongs here beside it. See runbooks/oauth2-proxy-auth-failures.md, which is the
+# by-hand equivalent an operator uses in the meantime.
+apiVersion: monitoring.coreos.com/v1
+kind: PrometheusRule
+metadata:
+  name: oauth2-proxy-auth-visibility
+  namespace: oauth2-proxy
+  labels:
+    release: kube-prometheus-stack     # kube-prometheus-stack's ruleSelector
+    app.kubernetes.io/name: oauth2-proxy
+spec:
+  groups:
+    - name: oauth2-proxy.auth-visibility
+      rules:
+        - alert: OAuth2ProxyGateUnobserved
+          # A namespace runs an oauth2-proxy gate container, and Prometheus holds no scrape
+          # target for that container in that namespace.
+          #
+          # BOTH SIDES ARE ALREADY-SCRAPED FAMILIES, deliberately:
+          #   * kube_pod_container_info  — kube-state-metrics, the declared owner in
+          #     alert-metrics-have-scrape-targets. It sees the container whether or not the
+          #     container exports anything, which is exactly the property needed here.
+          #   * up                       — UNIVERSAL. `up` carries the `container` label for any
+          #     ServiceMonitor/PodMonitor target, so this works for a future per-app scrape
+          #     without knowing what job name it will pick.
+          # Nothing in this expression touches `oauth2_proxy_*`, so the rule cannot itself be
+          # inert for the reason it is reporting.
+          #
+          # MEASURED ON PROD 2026-09-30 — this expression returns six namespaces right now:
+          #   {namespace="crafty"} {namespace="jackett"} {namespace="pihole"}
+          #   {namespace="qbittorrent"} {namespace="radarr"} {namespace="sonarr"}
+          # and excludes namespace="oauth2-proxy", whose two replicas ARE scraped. It fires on
+          # the state the cluster is in; it is not a rule waiting for a hypothetical.
+          expr: |
+            count by (namespace) (kube_pod_container_info{container="oauth2-proxy"})
+              unless
+            count by (namespace) (up{container="oauth2-proxy"})
+          # 1h, not 5m: a rollout briefly has a running sidecar whose target has not been
+          # discovered yet, and this condition is a standing architectural gap, never an
+          # emergency. Long enough that no deploy can produce it.
+          for: 1h
+          labels:
+            severity: warning
+          annotations:
+            summary: "oauth2-proxy gate in {{ $labels.namespace }} exports no metrics"
+            description: >-
+              The {{ $labels.namespace }} namespace runs an oauth2-proxy gate container and
+              Prometheus has NO scrape target for it, so every authentication decision that gate
+              makes — every redirect to Keycloak, every 403 on a group check, every session that
+              silently fails to hold — is invisible to metrics and to alerting. This is not a
+              theoretical gap: on 2026-09-30 the jackett gate answered a burst of the app's own
+              XHR calls with 302s to Keycloak, re-established a session, and lost it again 26
+              seconds later, and the only record anywhere was a line in the sidecar's stdout.
+              No alert could have fired, because there is nothing to alert on. Until a scrape
+              exists the runbook's LogQL queries are the substitute; the fix is
+              `--metrics-address=0.0.0.0:44180` on the sidecar plus a `metrics` port and a
+              ServiceMonitor, after which the auth-churn rule becomes writable.
+            runbook_url: "https://${BASE_DOMAIN}/runbooks/oauth2-proxy-auth-failures"