Adds WithTracing(serviceName) option using the official otelgin
instrumentation (go.opentelemetry.io/contrib/.../otelgin v0.65.0).
Each request produces a span with http.request.method, http.route,
and http.response.status_code attributes. Trace context is propagated
via W3C traceparent headers.
Also exposes NewTracerProvider() convenience helper for wiring up
a synchronous TracerProvider in tests and simple deployments.
Co-Authored-By: Virgil <virgil@lethean.io>
