title: How can I track users in Firebase Analytics?
body: <p>I want to track users separately or find out which users are doing an specific event<br>
  It will be terrible if I do this with creating custom audience for based on userId or some user property.<br>
  I'm looking for a efficient way for find my top clients and send them push notification or save their email.  </p>
  
  <p>Thanks in advance</p>