Function build_subscriber 

pub fn build_subscriber(
    config: &TelemetryConfig,
) -> Result<Subscriber, TelemetryError>

Construct the estate subscriber described by config without installing it.

This is the exact stack Telemetry::init installs — the seam used by the init-cost bench and by applications that must compose extra layers before installing. The OTLP tracer provider handle is not retained, so shutdown flushing is unavailable on this path; use Telemetry::init for the managed pipeline.

Errors

Same conditions as Telemetry::init minus TelemetryError::AlreadyInitialized (nothing global is touched).