grafana: the published vhost refuses /metrics

Grafana runs with metrics.enabled and no basic_auth_*, so its Prometheus
exposition was served to any anonymous client of grafana.<swarm> through
the published vhost's `location "/"`. Add an exact-match `= /metrics`
location returning 403 there. The loopback `grafana-metrics` vhost and
the collector's scrape of it are unchanged.

Comments on the metricsPort option and the loopback vhost now state that
the published vhost refuses /metrics; the gateway doc says the same.
A module-eval case pins the refusing location and that `/` still proxies.

docs/networking/gateway.md

@@ -52,7 +52,7 @@ The gateway always terminates TLS. Which certificate it serves depends on what y
 
 `tls.certDir` together with `tls.acme.enable` fails an assertion — pick one.
 
-⚠️ One vhost class has no TLS: a vhost bound to loopback for a local consumer. `grafana-metrics` (`nix/host-modules/swarm-grafana.nix`) is the one in the tree: it listens on `127.0.0.1` only and serves `= /metrics` from grafana's unix socket for this host's collector. Everything with a routable name follows the table above.
+⚠️ One vhost class has no TLS: a vhost bound to loopback for a local consumer. `grafana-metrics` (`nix/host-modules/swarm-grafana.nix`) is the one in the tree: it listens on `127.0.0.1` only and serves `= /metrics` from grafana's unix socket for this host's collector. The published `grafana.<swarm>` vhost answers `/metrics` with 403. Everything with a routable name follows the table above.
 
 ### ACME / Let's Encrypt (`tls.acme`)
 

nix/host-modules/swarm-grafana-service.nix

@@ -72,7 +72,9 @@ in
         {option}`services.hyperhive.swarm.otel.scrapeTargets` has: those
         targets are trusted by *proximity* rather than by credential.
         Deliberately **not** the published `grafana.<domain>` vhost, which
-        would put an authorization decision in front of a scrape.
+        would put an authorization decision in front of a scrape: that vhost
+        answers `/metrics` with 403, because Grafana serves it without
+        authentication.
 
         The number itself is arbitrary and free today;
         `state/eval-port-collisions.sh` is what keeps it that way, since a

nix/host-modules/swarm-grafana.nix

@@ -458,6 +458,13 @@ in
     services.nginx.virtualHosts."${cfg.domain}" = (gatewayCfg.lib.tlsFor cfg.domain) // {
       listen = gatewayCfg.lib.listen;
       extraConfig = gatewayCfg.lib.securityHeaders;
+      # Grafana serves its Prometheus exposition to anyone, signed in or not
+      # (`metrics.enabled` below, no `basic_auth_*`), so the published name
+      # refuses it; the scrape goes through the loopback `grafana-metrics`
+      # vhost instead. `=` is enough: nginx matches the decoded, normalised
+      # path, so `//metrics`, `/./metrics`, `/%6detrics` and `/metrics?x` all
+      # land here, and Grafana itself answers `/metrics/` with a login redirect.
+      locations."= /metrics".return = "403";
       locations."/" = {
         proxyPass = "http://unix:${socketPath}:/";
         proxyWebsockets = true;
@@ -484,7 +491,8 @@ in
     # ⚠️ `= /metrics` is an EXACT match, not a prefix: it is the difference
     # between exposing one endpoint and re-serving the whole of Grafana on an
     # unauthenticated port. Widening it would be a silent authorization bypass,
-    # since nothing else about this block would have to change.
+    # since nothing else about this block would have to change. The published
+    # vhost above refuses `/metrics`, so this listener is the only way to it.
     services.nginx.virtualHosts."grafana-metrics" = {
       listen = [
         {

nix/module-eval/grafana.nix

@@ -121,6 +121,21 @@ let
           grafanaOldPath.services.nginx.virtualHosts."grafana-metrics".locations."= /metrics".proxyPass
           == "http://unix:/run/swarm-grafana/grafana.sock:/metrics";
     }
+    {
+      # Grafana serves `/metrics` unauthenticated, so the published name must
+      # answer it itself; the scrape reads the loopback vhost above instead.
+      # `=` is what catches the normalised variants and still leaves the rest
+      # of Grafana proxied.
+      name = "grafana's published vhost refuses /metrics and still proxies everything else";
+      ok =
+        let
+          g = grafanaOldPath.services.hyperhive.swarm.grafana;
+          locs = grafanaOldPath.services.nginx.virtualHosts.${g.domain}.locations;
+        in
+        locs."= /metrics".return == "403"
+        && locs."= /metrics".proxyPass == null
+        && lib.hasPrefix "http://unix:" locs."/".proxyPass;
+    }
     {
       # This fixture enables grafana and NOT authelia, so the login-form toggle
       # has to follow "grafana requires SSO" rather than "both services are on