dd-trace for Node: npm install dd-trace, require and init it before any app code

# dd-trace for Node: install it, then require it FIRST

The Node tracer has one hard rule: it must be loaded before the modules it instruments. Get the order wrong and you get a running app with zero traces and no error.

## Setup

```sh
npm install dd-trace
```

At the very top of your entry file, before everything else:

```js
const tracer = require('dd-trace').init({
  service: 'my-service',
  env: 'prod',
  version: '1.2.3'
})
```

That is the whole integration for auto-instrumented frameworks (Express, Fastify, Nest, pg, redis, and the rest). The tracer patches modules as they load, which is why it must be first.

## The traps, in order of frequency

1. **Init after imports.** If your entry file imports express before requiring dd-trace, express never gets patched. Move the tracer require to line one. In ESM, use `--import dd-trace/register` or the loader hook so it runs before your code.
2. **No service/env/version.** Pass them to init or set `DD_SERVICE`, `DD_ENV`, `DD_VERSION`. Defaults guess and your service catalog turns to mush.
3. **Agent unreachable.** Default is the agent on the same host. In containers set `DD_AGENT_HOST` to the agent, or `DD_TRACE_AGENT_URL` to the full URL. Connection refused here is silent, traces just never arrive.
4. **Wrong site.** `DD_SITE` for non-US1 orgs, same as everything else Datadog.
5. **Major version jumps.** dd-trace has breaking changes across majors (the docs ship a migration guide for 0.x through 5.x). Pin the major in package.json.

## Verify

Start the app, hit an endpoint, check APM, Services. Also watch the tracer debug log on startup: it logs the agent endpoint it is using. If it says the wrong host, your env var did not take.

Serverless note: on Lambda, dd-trace works with the Datadog extension layer, but the init-first rule still applies inside the handler module.