Implement feature negotiation for CI job telemetry (Rails side)

Summary

Implement the Rails side of feature negotiation for CI job telemetry. When a Runner requests a job, the Rails response should indicate whether the job_telemetry feature is available for that project.

Job payload change

Add a job_telemetry entry to the existing features object in the /api/v4/jobs/request response:

{
  "features": {
    "job_telemetry": true
  }
}

When job_telemetry is absent or false, the Runner skips all telemetry instrumentation for that job.

This is a binary enablement signal — it does not carry sampling configuration or an OTEL Collector endpoint URL:

• Sampling is handled entirely at the OTEL Collector level using the probabilistic sampler processor, adjustable without Rails or Runner changes
• The OTEL Collector endpoint is statically configured on each runner manager (see architecture document)

Requirements

1. Add job_telemetry to GitlabFeatures 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. Only include trace_context (see #590587) when job_telemetry is enabled

Architecture Reference

• Job payload changes
• GitLab Runner changes

Related

• Trace context initialization (Rails): #590587
• Runner-side implementation: gitlab-runner#39231
• Coordinate with grouprunner core on the payload schema

Parent Epic

&20945