In general, custom metrics are defined as any single, unique combination of a metric name, host, and tag that are sent using statsd, dogstatsd, via API, or through extensions made to the Datadog agent. (Further details with examples on how we count a custom metric can be found here) However, certain standard integrations can also emit custom metrics as specified below.
Standard integrations that are limited to 350 metrics by default and emit custom metrics: ActiveMQ_XML & Go_expvar. If you'd like to increase this limit, please email firstname.lastname@example.org.
Other standard integrations that don’t have a default limit set and also emit custom metrics: SNMP, agent_metrics, directory, linux_proc_extras, nagios, win32_event_log & wmi.