Function set_global_span_exporter 

pub fn set_global_span_exporter(
    exporter: Box<dyn SpanExporter + Send + Sync>,
) -> bool

Install a process-wide span exporter.

Only the first call takes effect. Runtime and scheduler paths use emit_span after this has been installed. The global exporter is intentionally process-wide, mirroring the event sink: callers that need per-subsystem routing should install a multiplexer exporter.