If you run the following to browse the resource-description

subsystem=modcluster/mod-cluster-config=configuration/:read-resource-description(operations=true,inherited=false,recursive=false,proxies=false)

You'll encounter Object type = PROPERTY

"property" => {
"description" => "Properties for the metric.",
"type" => PROPERTY,
"required" => false
}

Which has an acceptable list of property values as described here.
http://docs.jboss.org/mod_cluster/1.2.0/html/java.AS7config.html#LoadMetric

There is no way to query the acceptable and currently supported list of property values via cli. You must hard code and risk going out of synch with as7/modclusters supported values.