code: func FromContext(ctx context.Context) (*Tracing, error) {
  	if ctx == nil {
  		panic("nil context")
  	}
  
  	tracer, ok := ctx.Value(tracingKey).(*Tracing)
  	if !ok {
  		return nil, errors.New("unable to find tracing in the context")
  	}
  	return tracer, nil
  }
docs: // FromContext Gets Tracing from context.