feat(obs): golden-signal metrics on /metrics + Prometheus scrape + Grafana dashboard (S-16c, refs #124)

Wire OTel metrics into the four remaining .NET services (acl, domain, event-subscriber,
projection-api) exactly as the BFF: ASP.NET Core + HttpClient instrumentation + the built-in
System.Runtime meter, exposed at /metrics via the Prometheus AspNetCore exporter (ADR-0024).
Prometheus scrapes one job per service; Grafana ships a pre-built 'Request path — golden
signals' dashboard (traffic/errors/latency/saturation). A verify-metrics CI step proves the
endpoints are scraped end to end.

infra/observability/grafana/Dockerfile

@@ -1,4 +1,4 @@
-# Grafana with datasources baked in via provisioning (S-16a, ADR-0023).
-# Dashboards (S-16c, #124) are added under provisioning/dashboards later.
+# Grafana with datasources + the golden-signals dashboard baked in via provisioning
+# (S-16a/S-16c, ADR-0023). Everything under provisioning/ is copied in below.
 FROM grafana/grafana:11.3.0
 COPY provisioning/ /etc/grafana/provisioning/

infra/observability/grafana/provisioning/dashboards/dashboards.yaml

@@ -0,0 +1,13 @@
+# Dashboard provider (S-16c, ADR-0023): Grafana loads every *.json in this folder as a
+# read-only, code-owned dashboard. The golden-signals board is versioned here, not
+# clicked together in the UI.
+apiVersion: 1
+
+providers:
+  - name: register-referentie
+    type: file
+    disableDeletion: true
+    allowUiUpdates: false
+    options:
+      path: /etc/grafana/provisioning/dashboards
+      foldersFromFilesStructure: false

infra/observability/grafana/provisioning/dashboards/golden-signals.json

@@ -0,0 +1,87 @@
+{
+  "uid": "golden-signals",
+  "title": "Request path — golden signals",
+  "tags": ["s-16c", "golden-signals"],
+  "timezone": "browser",
+  "schemaVersion": 39,
+  "version": 1,
+  "editable": true,
+  "refresh": "10s",
+  "time": { "from": "now-15m", "to": "now" },
+  "templating": {
+    "list": [
+      {
+        "name": "job",
+        "type": "query",
+        "datasource": { "type": "prometheus", "uid": "prometheus" },
+        "query": "label_values(http_server_request_duration_seconds_count, job)",
+        "includeAll": true,
+        "multi": true,
+        "current": { "text": "All", "value": "$__all" },
+        "refresh": 2
+      }
+    ]
+  },
+  "panels": [
+    {
+      "id": 1,
+      "title": "Traffic — requests/sec",
+      "type": "timeseries",
+      "datasource": { "type": "prometheus", "uid": "prometheus" },
+      "gridPos": { "h": 8, "w": 12, "x": 0, "y": 0 },
+      "fieldConfig": { "defaults": { "unit": "reqps", "custom": { "drawStyle": "line", "fillOpacity": 10 } }, "overrides": [] },
+      "targets": [
+        {
+          "refId": "A",
+          "expr": "sum by (job) (rate(http_server_request_duration_seconds_count{job=~\"$job\"}[$__rate_interval]))",
+          "legendFormat": "{{job}}"
+        }
+      ]
+    },
+    {
+      "id": 2,
+      "title": "Errors — 5xx responses/sec",
+      "type": "timeseries",
+      "datasource": { "type": "prometheus", "uid": "prometheus" },
+      "gridPos": { "h": 8, "w": 12, "x": 12, "y": 0 },
+      "fieldConfig": { "defaults": { "unit": "reqps", "custom": { "drawStyle": "line", "fillOpacity": 10 }, "color": { "mode": "fixed", "fixedColor": "red" } }, "overrides": [] },
+      "targets": [
+        {
+          "refId": "A",
+          "expr": "sum by (job) (rate(http_server_request_duration_seconds_count{job=~\"$job\",http_response_status_code=~\"5..\"}[$__rate_interval]))",
+          "legendFormat": "{{job}}"
+        }
+      ]
+    },
+    {
+      "id": 3,
+      "title": "Latency — p95 request duration",
+      "type": "timeseries",
+      "datasource": { "type": "prometheus", "uid": "prometheus" },
+      "gridPos": { "h": 8, "w": 12, "x": 0, "y": 8 },
+      "fieldConfig": { "defaults": { "unit": "s", "custom": { "drawStyle": "line", "fillOpacity": 10 } }, "overrides": [] },
+      "targets": [
+        {
+          "refId": "A",
+          "expr": "histogram_quantile(0.95, sum by (job, le) (rate(http_server_request_duration_seconds_bucket{job=~\"$job\"}[$__rate_interval])))",
+          "legendFormat": "{{job}} p95"
+        }
+      ]
+    },
+    {
+      "id": 4,
+      "title": "Saturation — CPU cores in use",
+      "type": "timeseries",
+      "datasource": { "type": "prometheus", "uid": "prometheus" },
+      "gridPos": { "h": 8, "w": 12, "x": 12, "y": 8 },
+      "fieldConfig": { "defaults": { "unit": "none", "custom": { "drawStyle": "line", "fillOpacity": 10 } }, "overrides": [] },
+      "targets": [
+        {
+          "refId": "A",
+          "expr": "sum by (job) (rate(dotnet_process_cpu_time_seconds_total{job=~\"$job\"}[$__rate_interval]))",
+          "legendFormat": "{{job}}"
+        }
+      ]
+    }
+  ]
+}

infra/observability/prometheus/prometheus.yml

@@ -1,6 +1,7 @@
-# Prometheus scrape config (S-16a, ADR-0023). For the backplane slice it scrapes
-# only itself; the .NET services' /metrics scrape targets are added in S-16c
-# (#124) when the services expose metrics.
+# Prometheus scrape config (S-16c, ADR-0023). Each .NET service exposes OTel metrics
+# at /metrics (Prometheus text format); one scrape job per service, so the service is
+# identified by the `job` label in the golden-signal dashboard. Targets are reached by
+# compose service name on the shared `cg` network (internal port 8080).
 global:
   scrape_interval: 15s
 
@@ -8,3 +9,19 @@ scrape_configs:
   - job_name: prometheus
     static_configs:
       - targets: ['localhost:9090']
+
+  - job_name: acl
+    static_configs:
+      - targets: ['acl:8080']
+  - job_name: domain
+    static_configs:
+      - targets: ['domain:8080']
+  - job_name: bff
+    static_configs:
+      - targets: ['bff:8080']
+  - job_name: event-subscriber
+    static_configs:
+      - targets: ['event-subscriber:8080']
+  - job_name: projection-api
+    static_configs:
+      - targets: ['projection-api:8080']