feat: pass logger/child logger as param to mixin

[mmarchini]

Passing the logger or child logger as a parameter to mixin allows users to set logger-specific context in the logger object that can be used by mixin to enrich the context to be added to the resulting JSON.

One example use case for this is avoiding the "duplicate keys" caveat that comes with child loggers. If the user wants to make a known key "mergeable", they could add a custom function that concatenates values to the key and then use mixin to pass that merged value to the context. For example:

instance = pino({
  mixin(obj, num, logger) {
    return {
      "tags": logger.tags
    }
  }
})
instance.tags = {}

instance.addTag = function (key, value) {
  child.tags[key] = value
}

function createChild (logger, ...context) {
  const child = logger.child(...context)
  child.tags = { ...logger.tags }
  child.addTag = function (key, value) {
    child.tags[key] = value
  }
  return child
}

instance.addTag('foo', 1)
child.addTag('bar', 2)
instance.info('this will only have `foo: 1`')
child.info('this will have both `foo: 1` and `bar: 2`')

[mcollina]

Good idea! Can you update the docs too ?

[mmarchini]

done!

[mcollina]

lgtm

[github-actions]

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