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## 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
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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/MeterListenerthat 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, andverify-metricsproves 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
/metricswith the standard OTel names (http_server_request_duration_seconds,dotnet_*), scraped straight by Prometheus. - No collector, no bespoke exposition code.
Negative / costs
- A
-betapackage in production services. Mitigated by the pin + theverify-metricsCI 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.