Function init 

pub fn init(config: &TelemetryConfig) -> Result<()>

Initialize OpenTelemetry tracing with OTLP export.

Sets up a global tracing subscriber that:

1. Emits human-readable logs to stderr (controlled by RUST_LOG).
2. Exports spans as OpenTelemetry traces via OTLP/gRPC.

Call shutdown before process exit to flush pending spans.