Customer can use default WF Metrics, MP Telemetry metrics, MicroMeter metrics. But AFAIU it's not recommended to use more then one metrics. Customers should be aware if they are using non-recommended configuration, so server should print the warning. See also WF Guide:

Note that this an alternative to the existing WildFly Metrics extension. While they may be run concurrently, it is not advisable, as this will likely have an impact on server performance due to the duplicated metrics collection.