## Summary Implement the Rails side of feature negotiation for CI job telemetry. When a Runner requests a job, the Rails response should include a `features.tracing` object that signals telemetry is enabled and provides trace context for that job. ## Job payload change Add a `tracing` entry to the existing `features` object in the `/api/v4/jobs/request` response: ```json { "features": { "tracing": { "trace_id": "<hex-encoded 16-byte trace ID derived from root pipeline ID>", "span_parent_id": "<hex-encoded 8-byte span ID of the trigger/bridge job, if child pipeline>", "otel_endpoints": ["https://otel-collector.gitlab.net/v1/traces"] } } } ``` - **Presence of `features.tracing`** = telemetry enabled; **absence** = telemetry disabled (no boolean flag needed). - `trace_id` is derived deterministically from the **root** pipeline ID, so all jobs across parent and child pipelines share the same trace. - `span_parent_id` is included only for child pipeline jobs and references the trigger (bridge) job's span ID, allowing child pipeline jobs to nest under the trigger job in the trace tree. This is a unified payload — sampling configuration is not included: - **Sampling** is handled entirely at the OTEL Collector level using the [probabilistic sampler processor](https://github.com/open-telemetry/opentelemetry-collector-contrib/tree/main/processor/probabilisticsamplerprocessor), adjustable without Rails or Runner changes - **The OTEL Collector endpoint** is provided from Rails via `features.tracing.otel_endpoints` (array, max 2) — first entry is a Rails application setting (see gitlab-org/gitlab#591941); optional second entry is a customer-configured BYO OTLP endpoint (project/group level, future). No static runner-side `config.toml` configuration needed. ## Requirements 1. Add `tracing` (with `trace_id` and optional `span_parent_id`) to the `features` object in the job payload response 2. Controlled by a **project-level feature flag** for gradual rollout - MVC: enable for DevExp Customer0 projects on hosted runners - Broader rollout: enable progressively for more projects 3. Include `trace_id` (derived from root pipeline ID) and `span_parent_id` (for child pipelines) inside `features.tracing` (see gitlab-org/gitlab#590587) ## Architecture Reference - [Job payload changes](https://handbook.gitlab.com/handbook/engineering/architecture/design-documents/ci_job_telemetry/#job-payload-changes) - [GitLab Runner changes](https://handbook.gitlab.com/handbook/engineering/architecture/design-documents/ci_job_telemetry/#gitlab-runner-changes) ## Related - Trace context initialization (Rails): gitlab-org/gitlab#590587 - Runner-side implementation: gitlab-org/gitlab-runner#39231 - Coordinate with ~"group::runner core" on the payload schema ## Parent Epic gitlab-org&20945