Hey Peter Braun great work and nice demo!

I think that Prometheus is useful for Mobile App Metrics: some of the data is time-series (e.g. SDK Version usage) and some of the data (e.g. Logs) is better suited for Elasticsearch. Fluentd can cover both scenarios, that's why I focused mostly on it.

I don't think we can eliminate the "Gateway" service completely because we also have non-time series data. But perhaps for time series data like you showed in the demo, clients may talk to Fluentd directly. This is something we can discuss.

Fluentd has a Prometheus output plugin

This is great. I wasn't even thinking about the possibility of this. Sounds like a good option to me, except the concern I described above.