code: func WithSharedSpans(val bool) TracerOption {
  	return func(o *Tracer) error {
  		o.sharedSpans = val
  		return nil
  	}
  }
docs: // WithSharedSpans allows to place client-side and server-side annotations
  // for a RPC call in the same span (Zipkin V1 behavior) or different spans
  // (more in line with other tracing solutions). By default this Tracer
  // uses shared host spans (so client-side and server-side in the same span).