Many great options have already been mentioned, but I add a few items, since I'm surprised they haven't been mentioned yet.
If you don't want/can't afford the human cost of managing your own Prometheus, _Datadog__ _is a nice managed solution which isn't free but could help you focus on your core features before you can put more energy into saving costs and move to Prometheus. Sine the ops team in my current and previous companies are very very small, this has proven to be a real key item in monitoring and debugging our stacks. (And since they bought logmatic last year, their log solution to complete the metrics and apm tools should be released at some point this year, which would make it even easier to get started and focus on developing the product)
Finally, if you want a quite long list of available quality tools, The Cloud Native Foundation regularly updates their landscape stuff, in which the Observability and Analysis section should help you making sure you consider the best options available. CloudNativeLandscape_v1.0.png and the Github repo if you want to track updates: github.com/cncf/landscape
Side note: Why is this tagged General Programming and don't have the devops or architecture tags? (this is not a rant, I often find myself missing question because I'm getting lost in tags... )