add metric: check_cluster_health_total&sync_operation_total

添加了两个指标，prometheus中显示如下：

# HELP apisix_ingress_controller_check_cluster_health Number of cluster health check operations
# TYPE apisix_ingress_controller_check_cluster_health counter
apisix_ingress_controller_check_cluster_health{controller_namespace="default",controller_pod="",name="default"} 1
# HELP apisix_ingress_controller_sync_success_total Number of sync operations
# TYPE apisix_ingress_controller_sync_success_total counter
apisix_ingress_controller_sync_success_total{controller_namespace="default",controller_pod="",resource="schema",result="success"} 1