code: function toBunyan (obj) {
    if (obj.msg && !obj.message) {
      obj.message = obj.msg
      delete obj.msg
    }
  
    if (typeof obj.level === 'number') {
      if (obj.level === 20) obj.level = 'debug'
      if (obj.level === 30) obj.level = 'info'
      if (obj.level === 40) obj.level = 'warn'
      if (obj.level === 50) obj.level = 'error'
    }
  }
docs: mutate a bole log to bunyan log obj -> null