Dd-trace-js: Request: Add support for Next.js

[Cretezy]

Next.js endpoint tracing would be very useful.

We have found some success using an Next.js custom server with Express and dd-trace, but since Next.js has it's own router to map paths to routes, it does not report the endpoint in Datadog, meaning it's only grouped by method.

[radykal-com]

We are facing the same problem

[mcuelenaere]

We are using the following workaround:

ddtrace.init(...)
  .use('express', {
    hooks: {
      request: (span, req, res) => {
        if (!span || !req || !res) {
          return;
        }

        let route;
        if (res.locals.route) {
          route = res.locals.route;
        } else if (req.path.startsWith('/static/')) {
          route = '/static/';
        } else if (req.path.startsWith('/_next/')) {
          route = '/_next/';
        } else {
          route = req.path;
        }

        span.setTag('resource.name', `${req.method} ${route}`);
      },
    },
  });
  ```

and then in `App.getInitialProps`, we have this:

```javascript
// keep track of route, used for sending to Datadog APM
res.locals.route = ctx.pathname;

[tcardonne]

This would be very useful!

We're also facing the same issue and investigating traces is very confusing due to the resource names only containing the method.

[fealaer]

We're also facing the same issue.

[laSinteZ]

@mcuelenaere I've tried your method but for some reason it didn't work for me. Could you tell me please if you are adding ddTraceJs using express-opentracing or something else?