The statistics for resource adapters are hard-coded to be disabled at startup:
https://github.com/wildfly/wildfly/blob/47830b44d1bd48c4fcdc2489cecaa3971c45c209/connector/src/main/java/org/jboss/as/connector/deployers/ra/processors/AbstractResourceAdapterDeploymentServiceListener.java#L91

It's not possible to enable the statistics in a persistent way - it needs to be done each time the servers starts e.g. via jmx.

Ideally there'd be a "statistics-enabled" attribute on the "<connection-definition>" element in the configuration file in the same way that there is on "<datasource>".