wip: feat(runtime): expose otel tracing layer under otel-exporter flag

[jonaro00]

Allows getting the otel layers used by feature "setup-otel-exporter" under feature "otel-exporter".

Usage:

let (layers, _guard) = shuttle_runtime::telemetry::otel_tracing_subscriber(
    env!("CARGO_CRATE_NAME"),
    env!("CARGO_PKG_VERSION"),
);
// registry
...
    .with(layers)
    ...
    .init();