Using custom pageprovider class breaks track usage in pp definition

Using a custom pp class for a pageprovider, the follwing message is displayed for no reason:

The page provider class 'org.nuxeo.labs.es.ext.pp.ExtendedElasticSearchNxqlPageProvider' should extend the Elasticsearch one. 

Moreover, the generated XML doesn't contain the <trackUsage>true</trackUsage> element even if  Generate Usage Statistics is checked