Fastify: 404s logged as errors?

[danhaller]

Describe the bug
Hi,

I've just upgraded to 0.13.0 with fastify support and was surprised to see datadog apm showing 404s as errors:

Fastify does create an error object but it goes on to return a response with a 404 status code https://github.com/fastify/fastify/blob/master/lib/fourOhFour.js#L53

Environment

• Operation system: https://github.com/nodejs/docker-node/blob/7e47b378c42b03ae6afae704c5bf5b724aae2b92/8/alpine/Dockerfile
• Node version: 8
• Tracer version: 0.13.0
• Agent version: