using datadog these days, newrelic previously - basically every metric you can collect. Disk i/o and network i/o is particularly important but most of the info

using datadog these days, newrelic previously - basically every metric you can collect. Disk i/o and network i/o is particularly important but most of the information you truly care about lies in application traces and application logs. Database metrics in a close second particularly cache/index usage and disk activity, query profiling. Network too if your application is bandwidth heavy.