This change exposes more metrics from the embedded Alertmanager, which are
valuable for troubleshooting Alertmanager operation particularly in HA setups.
```
grafana_alerting_notifications_total
grafana_alerting_notifications_failed_total
grafana_alerting_notification_requests_total
grafana_alerting_notification_requests_failed_total
grafana_alerting_notification_latency_seconds
grafana_alerting_nflog_gc_duration_seconds
grafana_alerting_nflog_snapshot_duration_seconds
grafana_alerting_nflog_snapshot_size_bytes
grafana_alerting_nflog_queries_total
grafana_alerting_nflog_query_errors_total
grafana_alerting_nflog_query_duration_seconds
grafana_alerting_nflog_gossip_messages_propagated_total
grafana_alerting_dispatcher_aggregation_groups
grafana_alerting_dispatcher_alert_processing_duration_seconds
```
Note that `alertmanager_dispatcher_aggregation_group_limit_reached_total` is
explicitly not exposed, as the group limit metrics are not enabled.