Or if you want to cover existing users, just ask during the first interactive invocation, when there is no prior user chosen setting (works for new installs too)...

pseudocode:

  choice = no
  if $brew_prefix/etc/homebrew.yaml:
    choice = read analytics $brew_prefix/etc/homebrew.yaml
  else:
    if interactive-tty and cmd != "--prefix":
      choice = ask-user "enable anonymous analytics (it helps us!)? Y/N: "
      write analytics=<choice> $brew_prefix/etc/homebrew.yaml
  if choice == yes:
    enable-analytics