egg项目中如何开启阿里性能平台的 Trace 慢链路追踪
## Background
项目依赖egg框架，用alinode做的性能监控，alinode已开启，慢http日志已经有数据了，Trace一直无数据，按照阿里提供的文档https://help.aliyun.com/document_detail/72715.html，装入了文档里面的依赖，因为项目没有使用express，按照文档实例代码仿写一直未成功，请问有没有egg项目开启 Trace 的例子代码

## Proposal
这是在一个接口里面仿写的一段代码
`async all_timeout() {
    const { ctx } = this;

    let req=this.ctx.request;
    req.parentSpan = tracer.startSpan('根模块');
    req.parentSpan.setTag(opentracing.Tags.PEER_HOSTNAME, req.hostname);
    req.parentSpan.setTag(opentracing.Tags.HTTP_METHOD, req.method.toUpperCase());
    req.parentSpan.setTag(opentracing.Tags.HTTP_URL, req.url);

    let res =await ctx.service.repository.getAll(3000)

    req.parentSpan.setTag(opentracing.Tags.HTTP_STATUS_CODE, res.statusCode);
    req.parentSpan.finish(req);

    let spanName = `${ctx.method} ${ctx.url}`;
    const spanOptions = {

      tags: {
  
        [opentracing.Tags.HTTP_METHOD]: ctx.method,
  
        [opentracing.Tags.HTTP_URL]: ctx.href,
  
      },
      
    };
    const span = tracer.startSpan(spanName, spanOptions)
    span.setTag('span.kind', 'server');
    span.finish(req);
    this.ctx.body=res
  }`

按此方法执行控制台会提示 span.finish without request will cause some problem, please use "span.finish(req)..."


