context.go
func NewContext(ctx context.Context, family, title string) (xtr.Trace, context.Context) {
	sp := parentOrChildFromContext(ctx, family, title)
	return sp, contextWithTrace(ctx, sp)
}
func NewContext(ctx context.Context, family, title string) (xtr.Trace, context.Context) {
	sp := parentOrChildFromContext(ctx, family, title)
	return sp, contextWithTrace(ctx, sp)
}
// NewContext returns a new context.Context and Trace with the given family and title.  The trace will
// be stored in the context.