With a redeploy of the webapplication to tomcat: This error message appears:
SEVERE: The web application [/MongoReporter] created a ThreadLocal with key of type [com.mongodb.DBTCPConnector$1] (value [com.mongodb.DBTCPConnector$1@6c747b55]) and a value of type [com.mongodb.DBTCPConnector.MyPort] (value [com.mongodb.DBTCPConnector$MyPort@534d4113]) but failed to remove it when the web application was stopped. Threads are going to be renewed over time to try and avoid a probable memory leak.

uploaded simple webapp to demonstrate