The problem is there are piecemeal solutions to the several of the parts of this problem, but not all, and some of them are really hard to meaningfully self-host without significant risk or complexity (ie. full text search engine). I'd personally love to build something like this, but I absolutely would sell yearly licenses and not have it be fully open source.

Look at ntfy.sh. From the website:

ntfy (pronounced notify) is a simple HTTP-based pub-sub notification service. It allows you to send notifications to your phone or desktop via scripts from any computer, and/or using a REST API. It's infinitely flexible, and 100% free software.

I thought someone was working on an open source Datadog. Forget what that was

Prometheus? Grafana?

(shameless plug) maybe it was us?

https://github.com/hyperdxio/hyperdx

Where is your cutoff for lightweight?

I personally am a big fan of self-hosting Sentry via their Helm chart. It's quite easy if you know your way around Kubernetes, but it probably also doesn't qualify as lightweight.