Have you already looked at fluentd or Graylog ? If you already looked at these, could you tell why they don't fit your needs and what else you're looking for? (Graylog as alerts and threshold and many many features) If I didn't get the question correctly and you're more interested in metrics rather than text logs, Prometheus is the open-source tool to look into. (Icinga2 is also worth being mentioned, as well as the InfluxDB/TICK stack which is really great for metrics!)
There are many log services out there, but if you want to keep control on everything in your stack, they would be my first guesses. I never (yet) had the opportunity to test fluentd for real, but it's a Linux Foundation/Cloud Native Computing Foundation project and looks quite solid. Graylog is really nice (or your own ELK stack, but might might require more ops work?)
Jan Vladimir Mostert
Idea Incubator