* Alerting: Add metrics for active receiver and integrations
Introduces metrics that allows us to track the number of configured receivers and integration in the Alertmanager for all orgs.
As a bonus, I realised that the alert reception metrics where not being exported nor collected. This does that too.
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.
