feat(integration): add fastify to the integration test suite

[domharrington]

🧰 Changes

Building on the work over in #528 - adding fastify to the integration test suite.

🧬 QA & Testing

Do the tests pass? ✅

[erunion]

What's the difference between the payload in onResponse hooks versus the one in onSend?

[domharrington]

Yeah great question, and it took me a good couple of hours to figure out the right hook to use here. onSend is called right before any response has been returned, so the data is there but there's no content-length header set up yet, so the har file has a content.size of 0:

metrics-sdks/packages/node/src/lib/process-response.ts

Line 68 in c093e21

size: Number(fixHeader(res.getHeader('content-length') || 0)),

onResponse is called after the response has been sent, but you can't access the body at that point... hence why we've gotta attach the payload in the onSend, then process it after the response has gone down.

The reason why we have to pass request.raw to the log function is because fastify's Request wrapper doesn't have request.httpVersion so this was coming through as HTTP/undefined.

The reason why we have to pass the Reply instance to the log function (as opposed to reply.raw) is also nuanced, but basically Node's ServerResponse object removes all headers from the object after it's been written to the response, so res.getHeader('content-length') would return with undefined when the response has finished: nodejs/node#10354

I hope this makes sense 😅 it didn't make a ton of sense to me tbh, hence why i'm hoping to modify the API design to make all of this stuff a little easier to figure out.