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feat(infra): observability backplane — Tempo + Prometheus + Grafana (S-16a, closes #122) (#125)
## What & why S-16a, the first of the **S-16 split** (#17 closed → #122/#123/#124, §13). Stands up a local, CI-friendly observability backplane so traces (S-16b) and metrics (S-16c) have somewhere to land, viewable in one Grafana. - **Grafana Tempo** — OTLP trace ingest (gRPC 4317 / HTTP 4318), local storage. - **Prometheus** — scrapes itself for now; service `/metrics` targets arrive in S-16c. - **Grafana** — Tempo + Prometheus datasources auto-provisioned with fixed uids (`tempo`, `prometheus`), exposed on :3000. All three are small **built images** with config baked in (`infra/observability/`), on the existing `cg` network. **No OTLP collector** (Tempo ingests OTLP directly; Prometheus scrapes) and **no config-volume seeding** — the tools aren't verbatim CG peer modules, so a 3-line `COPY` Dockerfile is the simpler path that still reaches sibling containers on the CI runner (**ADR-0023**). ### Verified, not assumed `make verify-observability` (new CI `verify-stack` step, run early) asks Grafana to reach both datasources — Prometheus via its health method, Tempo via the datasource proxy (Tempo's plugin implements no health method) — so it proves the datasources are wired, not merely that containers booted. Validated locally against the three containers (no external egress): Grafana healthy, both datasources reachable. Closes #122 ## Definition of Done - [x] Failing test committed first (`verify-observability` fails with no backplane). - [x] Implementation makes it pass; verified locally. - [x] Conventional Commits referencing the issue (`refs #122`). - [ ] CI green — awaiting Gitea Actions (verify-stack now includes the observability step; `docker compose config` validates locally). - [ ] `docker compose up` reaches green health within 3 min — new containers are lightweight and off the health-gate list. - [x] Docs — ADR-0023, demo-script, BACKLOG sync. - [x] ADR added — `docs/architecture/adr-0023-observability-stack.md`. - [x] Demo note in `docs/demo-script.md`. ## Notes for reviewers - **No app changes** — this is pure infra; the five services are untouched (instrumentation is #123/#124). - **Ports:** Grafana 3000 (admin/admin, anonymous viewer on), Prometheus 9090; Tempo internal to `cg`. - **CI:** the three containers are added to the failure log-dump list; deliberately **not** added to `WAIT_SVCS` (the check polls Grafana itself, so no in-image healthcheck tool is needed). Trades ~3 small image builds per run. - **Next:** #123 wires OTLP export + `AddAspNetCoreInstrumentation`/`AddHttpClientInstrumentation` into the five hosts so a request becomes one connected trace in Tempo. Reviewed-on: #125 |