I'm a huge fan of Sentry. But it is a tool for monitoring your own product.
Infra SaaS with great o11y is:
• Imagine that Planetscale gave you tons of metrics, events and traces about the queries you run in the DB that they manage"
• If Cloudflare workers gave you metrics, events, traces for the serverless code that they execute.
Unfortunately, I don't know anyone I'd describe as great.
The big vendors (AWS cloudwatch, GCP stackdriver) may be the best at that, but I want to believe that we, as an industry, can do better.