Alerting: Add tags to alert rules (#10989)

Ref #6552

docs/sources/alerting/notifications.md

@@ -167,26 +167,26 @@ Notifications can be sent by setting up an incoming webhook in Google Hangouts c
 
 ### All supported notifiers
 
-Name | Type | Supports images
+Name | Type | Supports images |Support alert rule tags
 -----|------------ | ------
-DingDing | `dingding` | yes, external only
-Discord | `discord` | yes
-Email | `email` | yes
-Google Hangouts Chat | `googlechat` | yes, external only
-Hipchat | `hipchat` | yes, external only
-Kafka | `kafka` | yes, external only
-Line | `line` | yes, external only
-Microsoft Teams | `teams` | yes, external only
-OpsGenie | `opsgenie` | yes, external only
-Pagerduty | `pagerduty` | yes, external only
-Prometheus Alertmanager | `prometheus-alertmanager` | yes, external only
-Pushover | `pushover` | yes
-Sensu | `sensu` | yes, external only
-Slack | `slack` | yes
-Telegram | `telegram` | yes
-Threema | `threema` | yes, external only
-VictorOps | `victorops` | yes, external only
-Webhook | `webhook` | yes, external only
+DingDing | `dingding` | yes, external only | no
+Discord | `discord` | yes | no
+Email | `email` | yes | no
+Google Hangouts Chat | `googlechat` | yes, external only | no
+Hipchat | `hipchat` | yes, external only | no
+Kafka | `kafka` | yes, external only | no
+Line | `line` | yes, external only | no
+Microsoft Teams | `teams` | yes, external only | no
+OpsGenie | `opsgenie` | yes, external only | no
+Pagerduty | `pagerduty` | yes, external only | no
+Prometheus Alertmanager | `prometheus-alertmanager` | yes, external only | yes
+Pushover | `pushover` | yes | no
+Sensu | `sensu` | yes, external only | no
+Slack | `slack` | yes | no
+Telegram | `telegram` | yes | no
+Threema | `threema` | yes, external only | no
+VictorOps | `victorops` | yes, external only | no
+Webhook | `webhook` | yes, external only | no
 
 # Enable images in notifications {#external-image-store}
 
@@ -197,6 +197,14 @@ Be aware that some notifiers requires public access to the image to be able to i
 
 Notification services which need public image access are marked as 'external only'.
 
+# Use alert rule tags in notifications {#alert-rule-tags}
+
+Grafana can include a list of tags (key/value) in the notification.
+It's called alert rule tags to contrast with tags parsed from timeseries.
+It currently supports only the Prometheus Alertmanager notifier.
+
+ This is an optional feature. You can get notifications without using alert rule tags.
+
 # Configure the link back to Grafana from alert notifications
 
 All alert notifications contain a link back to the triggered alert in the Grafana instance.