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## What & why S-16b, second of the S-16 split, on top of the #125 backplane. The five .NET services now emit OpenTelemetry traces so a request is **one connected trace** across them. - Each host wires `AddOpenTelemetry().WithTracing(...)` with `AddAspNetCoreInstrumentation` (incoming) + `AddHttpClientInstrumentation` (outgoing) + `AddOtlpExporter` to **Tempo**. - Because every cross-service call already goes through a typed `HttpClient` (§8 boundaries), the W3C `traceparent` propagates with no manual code — bff → domain → acl → openzaak and bff → projection-api stitch into a single trace. - Service name + OTLP endpoint come from `OTEL_*` env set per app service in compose. `/health` is filtered out so liveness polls don't flood the traces. No new ADR — ADR-0023 already records the stack + the two documented gaps (browser-side tracing is out of scope, so the trace begins at the BFF; the async Flowable-poll boundary is a separate trace). Closes #123 ## Definition of Done - [x] Failing test committed first (`verify-tracing` fails with no instrumentation). - [x] Implementation makes it pass — **validated locally end to end**: a real connected trace spanning `bff` + `projection-api` was found in Tempo (BFF→projection→db + Tempo subset, no OpenZaak/egress). - [x] Conventional Commits referencing the issue (`refs #123`). - [ ] CI green — awaiting Gitea Actions (verify-tracing added to verify-stack after verify-bff). - [x] `docker compose up` health unaffected — services boot healthy even when Tempo is unreachable (exporter no-ops; verified). - [x] Docs — demo-script + BACKLOG. - [x] ADR — none needed (covered by ADR-0023). ## Notes for reviewers - **Per-service wiring, no shared lib:** the block is duplicated across the five hosts by design — services don't share code across boundaries here (§8), same as the duplicated typed clients. - **Packages:** OpenTelemetry.Extensions.Hosting / Instrumentation.AspNetCore / Instrumentation.Http / Exporter.OpenTelemetryProtocol, all 1.17.0, pinned per-csproj (no central props file). - **The check** generates anonymous BFF→projection traffic (no auth, no OpenZaak), then queries Tempo (TraceQL search → fetch trace → assert both service.names present) from a python:3-slim container in-network — same idiom as run-projection-check.sh. - **Next:** #124 (S-16c) adds `/metrics` + Prometheus scrape targets + golden-signal Grafana dashboards. Reviewed-on: #126