The question you're asking isn't really about metadata, but rather who has custody if it. Your argument is not that WhatsApp is bad because it generates metadata --- WhatsApp leaks far less data to its provider than Telegram --- but rather than WhatsApp is bad because what metadata it generates goes to Facebook.

That's a fine, coherent argument. There are aspects of Scott's comment I disagree with too --- though I generally think most users are best served by WhatsApp.

If you don't want to use anything operated by Facebook, use Wire. Wire is based on Signal's double-ratchet model. It's encrypted by default. Wire's operators get much less data than Telegram's. Wire is operated out of Europe --- I don't think this matters but many people do.

The important thing is just that you not use Telegram for secure messaging. Telegram is deeply unserious about security. There are much better alternatives. If your friends want to use Telegram to decide which movie to see tonight, fine: I use Slack too, and Slack is probably only marginally more secure than Telegram. But don't use either of them for life-and-death secrets.