I'd love an open source alternative. But there just isn't one for APM (which is our main use case). Nothing comes close. Every time I see "OpenTelemetry integration" I just close the page. Hours and hours of manual setup, code pushes, etc while New Relic installs once and works.
I assume it's the same for people who use DataDog begrudgingly.
Depending on the language/environment/framework, OpenTelemetry Autoinstrumentation just works. It's the new standard, and lots of working is ongoing to make it work for everything, everywhere, and even the big observability vendors are adopting it.
Learning that datadog charges these as custom metrics was shocking. This opens a wormhole of allow-list or opt-in considerations, and then there is tag cardinality or even introducing a middleware like vector. It feels very backwards to spend effort on reducing observability.
If datadog was crap it would make things easier, but it really is a fantastic product, and a business after all. Prometheus integration is just so very cumbersome which is probably strategic I would imagine.
The "omg my bill is out of control" issue is usually manifest from a few sources, one of the biggest is relying heavily on custom metrics added over time, and so you think you're paying X but really you end up paying 2-3X or more by the end of the year. But the tricky thing is, most of those things that cost a lot of money either have a lot of value, or held a lot of value at the time.
(I work for a Datadog competitor)