# 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.