Add OTel distributed tracing support with zero overhead when disabled.
Configuration (plan §10):
- tracing.enabled: false (default, zero overhead)
- tracing.endpoint: "http://tempo.monitoring.svc:4317"
- tracing.service_name: "miroir"
- tracing.sample_rate: 0.1 (head-based sampling)
Span hierarchy:
- Parent: inbound request (POST /indexes/:index/search)
- Child: scatter plan construction
- Parallel children: one per node in covering set
- Child: merge operation
Resource attributes: service.name, service.version, host.name
When disabled (tracing.enabled: false), no OTel library calls are made.
Shutdown handler flushes pending traces before exit.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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