OpenTelemetry OTLP exporter wiring

[jeremymanning]

Parent: #7

Wire the OpenTelemetry OTLP exporter for traces and metrics when otel_endpoint is configured. The dependencies (opentelemetry, opentelemetry-otlp) are in Cargo.toml but not wired.

Files: src/telemetry/mod.rs line 20

[jeremymanning]

Resolved by PR #58 (spec 003-stub-replacement). All 77 tasks complete, 489+ tests passing, CI green on Linux/macOS/Windows.