Add metric rule_group_rules_loaded to get the number of rules loaded (#5090)
Add metric rule_group_rules_loaded to get the number of rules loaded (#5090)

Signed-off-by: Vishnunarayan K I <dcea562cbb1f9e152fa122a368061dff532affbc@gmail.com>
alileza/prometheus,Gouthamve/prometheus,prometheus/prometheus,GoogleCloudPlatform/prometheus,alileza/prometheus,kangwoo/prometheus,GoogleCloudPlatform/prometheus,GoogleCloudPlatform/prometheus,Gouthamve/prometheus,keegancsmith/prometheus,prometheus/prometheus,alileza/prometheus,kangwoo/prometheus,kangwoo/prometheus,prometheus/prometheus,keegancsmith/prometheus,prometheus/prometheus,Gouthamve/prometheus,kangwoo/prometheus,GoogleCloudPlatform/prometheus,prometheus/prometheus,alileza/prometheus,keegancsmith/prometheus,keegancsmith/prometheus,GoogleCloudPlatform/prometheus,keegancsmith/prometheus,alileza/prometheus,Gouthamve/prometheus