feat(obs): Prometheus metrics on /metrics + golden-signal Grafana dashboard (closes #124) (#129)

## What & why

S-16c, the last of the S-16 (#17) split, on top of the backplane (#122) and distributed tracing (#123). The five .NET services now expose OpenTelemetry **metrics** in Prometheus format at `/metrics`; Prometheus scrapes each (one job per service); and Grafana ships a pre-built **Request path — golden signals** dashboard (traffic / errors / latency / saturation), split by service.

Closes #124

### How

- Each service adds `.WithMetrics(AddAspNetCoreInstrumentation + AddHttpClientInstrumentation + AddMeter("System.Runtime") + AddPrometheusExporter)` and maps `/metrics`. Same shape as the S-16b tracing wiring already in these `Program.cs` files.
- `infra/observability/prometheus/prometheus.yml`: one scrape job per service (`acl`, `domain`, `bff`, `event-subscriber`, `projection-api`), reached by compose service name.
- `infra/observability/grafana/provisioning/dashboards/`: dashboard provider + `golden-signals.json` (baked into the Grafana image by the existing `COPY provisioning/`).
- `verify-metrics` (new CI verify-stack step + Makefile target): generates BFF traffic and asserts Prometheus scraped the golden-signal metric from every service. Mirrors `verify-tracing`.

### Dependency (CLAUDE.md §13/§14)

Adds `OpenTelemetry.Exporter.Prometheus.AspNetCore` `1.17.0-beta.1` (matched to the `1.17.0` core already in use). It gives the OTel-native `/metrics` pull endpoint; replacing it would mean hand-rolling Prometheus exposition over a `MeterListener`; the risk is that it is a **prerelease** package (the whole OTel .NET Prometheus line is `-beta`) — pinned, wired only in `Program.cs`, and gated by `verify-metrics`. Recorded in **ADR-0024**.

## Definition of Done

- [x] Linked Gitea issue (#124).
- [x] Failing test committed before the implementation (`test(bff): /metrics exposes http-server request duration`).
- [x] Implementation makes the test pass.
- [ ] CI green — pending Gitea Actions run.
- [x] `docker compose up` reaches green health within 3 min (backplane images unchanged in shape; not on the health gate, ADR-0023).
- [x] Docs updated — demo-script S-16c entry.
- [x] ADR added — ADR-0024.
- [x] Demo note in `docs/demo-script.md`.

## Notes for reviewers

- `/health` polls are counted as traffic (metrics aren't path-filtered, unlike traces). Fine for a demo dashboard and honest — real load stacks on top.
- `projection-api` has no Stryker config (unchanged); the four mutated services carry the metrics wiring in `Program.cs`, same as the merged S-16b tracing code.
- Metric names verified against a live service: `http_server_request_duration_seconds{,_bucket,_count}`, label `http_response_status_code`, `dotnet_process_cpu_time_seconds_total`.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed-on: #129

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}}"
+        }
+      ]
+    }
+  ]
+}