We definitely used datadog to monitor internal systems while I was there many years ago. Dogfooding was a celebrated practice and I'd be surprised to learn that it's changed.
I remember listening to Charity Majors[1] talk about this on a podcast -- in the case of Honeycomb they have a "dogfood" cluster than monitors production, and a "kibble" cluster that monitors the dogfood.
Cute, but it gets the point across: watchmen for the watchmen, with each layer slightly less mission-critical than the last.