feat(telemetry): add lightweight serving telemetry

[ClSlaid]

Summary

• add lightweight frontend telemetry with OpenInfer /metrics, optional request logs/traces, /openinfer/traces, and caller-owned OTLP payload export
• centralize sparse request lifecycle tracing and metrics facade counters at the shared engine TokenSink boundary
• keep success traces at DEBUG, rejected requests at WARN, execution failures at ERROR, and omit per-token tracing/metrics from the shared serving path
• keep reduce_request focused on output folding and add frontend/sim coverage for the telemetry routes

Tests

• cargo test --release -p openinfer-engine --lib
• cargo test --release -p openinfer-vllm-frontend --lib
• cargo test --release -p openinfer-sim --test frontend_e2e simulated_engine_serves_openai_completions_over_http -- --nocapture
• cargo fmt --check
• git diff --check