A custom metric can be created only for monitored logs (log files that are stored on the Dynatrace server). If you use Log Monitoring as an ad hoc troubleshooting tool for a specific process or host, you'll likely view host-specific logs that haven't been designated for monitoring (that is, stored). As a result, the custom metric feature isn't available for such logs.
Ok, I undesntand, but How do I save the log files in my Dynatrace Cluster Servers? Do I need a NFS?
I think no
I see in the log section the option for start monitoring the logs I need, but after I click this option I couldn't create the metric.