tracer.go
func (t *Tracer) Context(ctx context.Context) context.Context {
	return opentracing.ContextWithSpan(ctx, t.Last())
}
func (t *Tracer) Context(ctx context.Context) context.Context {
	return opentracing.ContextWithSpan(ctx, t.Last())
}
// Context returns a new context with the latest span stored as a reference for
// handlers that will call NewTracerFromRequest or similar.