Prerequisites

[✓] run npm run test && npm run benchmark --if-present
[✓] tests and/or benchmarks are included
[✓] documentation is changed or added
[✓] commit message and code follows the Developer's Certification of Origin and the Code of conduct

Proposal

Implement a customizable log management system through an encapsulated plugin

Description

Hello, I've been working on customizing the log management system which originally was derived from #4463.
As previously specified in #4463, we currently have a log management system set in place which only displays a default message, "incoming request", as shown below:

if (disableRequestLogging === false) {
  childLogger.info({ req: request }, 'incoming request')
}

The purpose of this PR is to allow the user to customize the log management system so they can include criteria and data that is compatible with their particular project.

For this particular feature, I have implemented a plugin approach rather than hard-coding the logger through route.js.
The objective of the plugin approach is to produce an object or plugin that is encapsulated by a childLogger object. This plugin will possess and inherit all the log management system properties it needs to create a customizable log management system. In addition, it can be reused and shared through several different Fastify projects.

Advantages

• Object is reusable

• Contains settings and properties directly adopted through the childLogger object. As a result, it has the capability to perform more functionalities and logic.

• The whole entire Log Management system is now encapsulated through an object. Therefore it makes it easier to perform functionalities. It’s also much more organized and maintainable than the previous approaches.

• Scalable and adaptable.

Modifications

fastify.js

Added customRequestLog factory option:

const customRequestLog = typeof options.customRequestLog === 'function'
  ? options.customRequestLog
  : (req) => 'incoming request'

Updated onBadUrl and buildAsyncConstraintCallback error paths:

if (disableRequestLogging === false) {
  childLogger.info({ req: request }, customRequestLog(request))
}

fastify.d.ts:

Added TypeScript type definition:

customRequestLog?: (request: FastifyRequest) => string,

examples/custom-logger-plugin.js

Added example plugin demonstrating hook-based logging:

function customLogger (fastify, options, done) {
  fastify.addHook('onRequest', (request, reply, done) => {
    request.log.info({
      req: request,
      timestamp: new Date().toISOString(),
      method: request.method,
      url: request.url,
      ip: request.ip
    }, 'Custom Request Log')
    done()
  })
  done()
}

docs/Reference/Server.md

Included documentation for customRequestLog option

Please tell me what you think. Thank you