code: def usage(self, startTime, endTime, vars=None, period=None,
                groupby=None, name=None, stype=None, etype=None,
                appId=None, deviceId=None, username=None, appOrgId=None,
                userOrgId=None, hostOrgId=None):
          
  
          url = self.root + "/usage"
          startTime = str(int(local_time_to_online(dt=startTime)))
          endTime = str(int(local_time_to_online(dt=endTime)))
  
          params = {
                      'f' : 'json',
                      'startTime' : startTime,
                      'endTime' : endTime,
                      'vars' : vars,
                      'period' : period,
                      'groupby' : groupby,
                      'name' : name,
                      'stype' : stype,
                      'etype' : etype,
                      'appId' : appId,
                      'deviceId' : deviceId,
                      'username' : username,
                      'appOrgId' : appOrgId,
                      'userOrgId' : userOrgId,
                      'hostOrgId' : hostOrgId,
                   }
  
          params = {key:item for key,item in params.items() if item is not None}
          return self._post(url=url,
                               param_dict=params,
                               securityHandler=self._securityHandler,
                               proxy_url=self._proxy_url,
                               proxy_port=self._proxy_port)
docs: returns the usage statistics value