do not repeat execute alert in grafana ha

Currently, if Grafana alerting HA is enabled, all alerts will be executed once on all Grafana machines. This can lead to high load on the underlying storage. Recommendation: Use a unique index in a database such as MySQL to ensure that only one Grafana database is running alerts. Example: Before execution, Grafana first checks if any other Grafana instances are running alerts in MySQL. If not, it creates a lock in MySQL using a unique index and then begins executing the Grafana alerts.