Child logger support

[jeffijoe]

Loving Bristol, and have even written a target + formatter myself. 😄

It would be awesome to have support for child loggers! Imagine creating a child logger for each HTTP request, then set some globals for it (such as the calling user, request info, etc). It would share the transports, formatters and globals of it's parent.

Example API:

const logger = new Bristol({ ... })

app.use(authenticate())
app.use((ctx, next) => {
  const child = logger.createChildLogger({ /*additional config for child */ })

  child.setGlobal('user_id', () => ctx.state.user.id)
  ctx.logger = child
  return next()
})

app.use(get('/todos', (ctx) => {
  ctx.logger.info('Getting todos')
}))

[jeffijoe]

As a temporary trick to accomplish this:

/**
 * Creates a child logger from the specified parent.
 *
 * @param parentLogger
 */
export function createChildLogger(parentLogger) {
  const child = new Bristol()
  child.setSeverities(Object.keys(parentLogger._severities))
  child._targets = [...parentLogger._targets]
  child._transforms = [...parentLogger._transforms]
  child._globals = { ...parentLogger._globals }
  const getGlobals = child._getGlobals
  child._getGlobals = function() {
    return {
      ...parentLogger._getGlobals(),
      ...getGlobals.call(this)
    }
  }
  return child
}