Enable calculating id based on http request

[tan-tan-kanarek]

Refs: #25

[puzpuzpuz]

@tan-tan-kanarek thanks for your contribution. Could you explain the use case behind this addition?

[tan-tan-kanarek]

I will share my use case, but I'm sure there are many other similar use cases.

var app = express();
// auth is adding session object to the req object.
app.use(auth, rTracer.expressMiddleware({
    requestIdFactory: req => {
        let userSid = req.session ? req.session.userSid : null;
        let requestId = req.headers['x-requestid'];
        let sequenceSid = req.headers['x-sequencesid'];

        if(!requestId) {
            requestId = Math.random().toString(36).substring(2, 12);
        }
        return {sequenceSid, requestId, userSid}; // I return object but it could be a calculated string as well
    }
}));```

Later in my logger transformer I use the user info, the sequence (session) info and request-id.
Instead of direct headers I could take the session-id from the request cookies or some other data from the query-string.

[puzpuzpuz]

I see. Let me review the changes.

[puzpuzpuz]

Could you also add tests to verify this change?

[puzpuzpuz]

Do you need any advice on tests? I see some changes, but they're not adding tests.

[tan-tan-kanarek]

Thank you, advice on tests will be appreciated.

[puzpuzpuz]

It could be something like the following test:

  test('calls request id factory with req', () => {
    const app = express()
    const idFactory = (req) => req

    app.use(rTracer.expressMiddleware({
      requestIdFactory: idFactory
    }))

    app.get('/test', (req, res) => {
      if (req === rTracer.id()) {
        res.end()
      } else {
        res.status(500).end()
      }
    })

    return request(app).get('/test')
      .then(res => {
        expect(res.statusCode).toBe(200)
      })
  })

One more thing to notice here is that the suggested implementation always passed request object into requestIdFactory. The default value of that option is uuidv1 and such behavior may lead to various issues with collisions between req and options expected by uuidv1. So, it's better to pass the object only when the requestIdFactory option is set by the user.

[puzpuzpuz]

Suggested change

	requestId = requestId || (requestIdFactory ? requestIdFactory(request.raw) : uuidv1())
	requestId = requestId || (requestIdFactory ? requestIdFactory(request) : uuidv1())

[tan-tan-kanarek]

The raw contains all the info one might need from the request and will be compatible between all versions of fastify.

[puzpuzpuz]

Other non-Express middlewares and plugins from this PR provide framework specific request objects, so having request.raw only in Fastify middleware doesn't seem to be consistent. Also, frameworks usually expose convenient APIs in their request wrapper objects, so to me it makes more sense to allow user to deal with the familiar API (which is framework's request wrapper).

[puzpuzpuz]

Closing this one in favor of #48