enable override for user_id

src/index.ts

@@ -4,7 +4,7 @@ import UAParser from 'ua-parser-js'
 // Get the user ID stored in the client, if it does not exist, make a random one, save it in the client, and return it.
 const getUserId = (event: MCEvent) => {
  const { client } = event
- let userId = client.get('user_id')
+ let userId = event.payload.user_id || client.get('user_id')
  if (!userId) {
  userId = crypto.randomUUID()
  client.set('user_id', userId, { scope: 'infinite' })