Exclude paths/routes from 503 errors

[melroy89]

Prerequisites

• [X] I have written a descriptive issue title
• [X] I have searched existing issues to ensure the feature has not already been requested

🚀 Feature Proposal

In some cases you do not want to trigger the "Service Unavailable" 503 error, a very good example would be the Swagger documentation page.

Meaning there should be an option for some kind of whitelist.

Motivation

Some pages or routes shouldn't be blocked, for example I would like to keep showing the Swagger Docs page, even if the healthCheck fails for instance.

Example

Just an proto-type idea for such an interface by introducing an option like routeWhitelist:

fastify.register(require('@fastify/under-pressure'), {
  maxEventLoopDelay: 1000,
  routeWhitelist: ['/docs'],
})

An alternative solution is to provide a config option to the route config settings for under-pressure. Just like you can set a log level per route, you should be able to "ignore" the route from under-pressure plugin.