With Piwik we would like to offer an open source Application Performance and Monitoring tool for every app developer and system administrator to benefit from. One of the goals will be to let users easily collect large number of metrics with a low resolution (eg. 10s or 1min).
What to track
When looking at defining good API for tracking custom server monitoring metrics, we could look at existing popular platforms that offer good APIs. For example I saw that Google launched this tool "Cloud Monitoring" http://googlecloudplatform.blogspot.ca/2015/01/Gain-insight-into-the-performance-of-your-apps-with-Google-Cloud-Monitoring_13.html
Cassandra Performance Dashboard You can also tailor Cloud Monitoring to suit your needs: you can publish custom metrics to our API and bring them together with system and infrastructure metrics on custom dashboards. which could be an interesting API to take a look at
Nothing new on this one?