gocontext.go
func ContextWithSpan(ctx context.Context, span Span) context.Context {
	return context.WithValue(ctx, activeSpanKey, span)
}
func ContextWithSpan(ctx context.Context, span Span) context.Context {
	return context.WithValue(ctx, activeSpanKey, span)
}
func ContextWithSpan ( ctx context . Context , span Span ) context . Context { return context . WithValue ( ctx , activeSpanKey , span ) }
// ContextWithSpan returns a new `context.Context` that holds a reference to
// `span`'s SpanContext.
ContextWithSpan returns a new context . Context that holds a reference to span s SpanContext .