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.

docs/architecture/adr-0024-prometheus-aspnetcore-exporter.md

@@ -0,0 +1,53 @@
+# ADR-0024: Expose OTel metrics with the (prerelease) Prometheus AspNetCore exporter
+
+- **Status:** Accepted
+- **Date:** 2026-07-24
+- **Deciders:** Respellion engineering
+- **Slice:** S-16c (#124), last of the S-16 (#17) split
+
+## Context
+
+ADR-0023 already fixed the shape of metrics collection: **Prometheus scrapes each
+service's `/metrics`** (pull, no collector). S-16c implements it. That needs a package
+that turns the OpenTelemetry `MeterProvider` into a Prometheus scrape endpoint inside
+ASP.NET Core. The canonical one is `OpenTelemetry.Exporter.Prometheus.AspNetCore`
+(`AddPrometheusExporter()` + `app.MapPrometheusScrapingEndpoint()`).
+
+The catch: that exporter has **never had a stable release** — the whole OTel .NET
+Prometheus exporter line is versioned `-beta` (we pin `1.17.0-beta.1`, matched to the
+`1.17.0` core we already use). Adding it is a new dependency (CLAUDE.md §14), and taking
+a prerelease package into all five services is the decision worth recording.
+
+## Decision
+
+**Add `OpenTelemetry.Exporter.Prometheus.AspNetCore` `1.17.0-beta.1` to the five .NET
+services and expose `/metrics` with it.**
+
+- What it gives us: the OTel-native pull endpoint, so the meters we already register for
+  tracing-adjacent instrumentation surface as Prometheus text with zero extra plumbing.
+- What we'd write to replace it: a hand-rolled `IMetricsListener`/`MeterListener` that
+  formats Prometheus exposition text — real work, and a reimplementation of a widely-used
+  library for no gain.
+- Risk it adds: a prerelease API that can shift between betas. Contained: it is only
+  wired in `Program.cs` (two calls per service, excluded from mutation), the version is
+  pinned, and `verify-metrics` proves the endpoint + scrape actually work each CI run.
+
+The alternative — pushing metrics over OTLP to a collector that re-exposes them — was
+already rejected in ADR-0023 (no collector hop). Not revisited here.
+
+## Consequences
+
+**Positive**
+
+- Golden-signal metrics on `/metrics` with the standard OTel names
+  (`http_server_request_duration_seconds`, `dotnet_*`), scraped straight by Prometheus.
+- No collector, no bespoke exposition code.
+
+**Negative / costs**
+
+- A `-beta` package in production services. Mitigated by the pin + the `verify-metrics`
+  CI gate; upgrading tracks the OTel core version bumps.
+
+## Coupling rules touched (CLAUDE.md §8)
+
+None. Metrics are passive: Prometheus pulls; no service calls into the stack.