Get metrics for each Spark RDD.

def _spark_rdd_metrics(self, instance, running_apps, addl_tags, requests_config):
        """
        Get metrics for each Spark RDD.
        """
        for app_id, (app_name, tracking_url) in iteritems(running_apps):

            base_url = self._get_request_url(instance, tracking_url)
            response = self._rest_request_to_json(
                base_url, SPARK_APPS_PATH, SPARK_SERVICE_CHECK, requests_config, addl_tags, app_id, 'storage/rdd'
            )

            tags = ['app_name:%s' % str(app_name)]
            tags.extend(addl_tags)

            for rdd in response:
                self._set_metrics_from_json(tags, rdd, SPARK_RDD_METRICS)

            if len(response):
                self._set_metric('spark.rdd.count', COUNT, len(response), tags)