Earlier quoted context omitted.
You have an incredulous tone, but you can host Sentry (it's open source) and it's very common to host databases in the cloud.
I just hoped that everything on that server could run locally and not depend on any cloud based tech.
I strongly suggest to read the requirements fully before commenting what amounts to misinformation at best.
You can run everything locally. You don't need spanner and can use mysql instead as a database. Also, as the previous commenter already told you, you can can run sentry locally.
Let's see whether you're willing and able to walk back your comments. I'm in the same boat as you - I'd want to run everything locally and I'm very happy to see it's entirely possible.