function identifyVisitor(id, traits = {}, conf = {}) {
  if (gaNotLoaded()) return
  const { instancePrefix } = getInstanceDetails(conf)
  if (id) ga(`${instancePrefix}set`, 'userId', id)
  if (Object.keys(traits).length) {
    const custom = formatObjectIntoDimensions(traits, conf)
    ga(`${instancePrefix}set`, custom)
  }
}
Identify a visitor by Id
@param  {string} id - unique visitor ID