Document kapacitor_alert and kapacitor_cluster measurements (#7278)

plugins/inputs/kapacitor/README.md

@@ -1,6 +1,6 @@
 # Kapacitor Plugin
 
-The Kapacitor plugin will collect metrics from the given Kapacitor instances.
+The Kapacitor plugin collects metrics from the given Kapacitor instances.
 
 ### Configuration:
 
@@ -23,12 +23,20 @@ The Kapacitor plugin will collect metrics from the given Kapacitor instances.
   # insecure_skip_verify = false
 ```
 
-### Measurements & Fields
+### Measurements and fields
 
 - [kapacitor](#kapacitor)
     - [num_enabled_tasks](#num_enabled_tasks) _(integer)_
     - [num_subscriptions](#num_subscriptions) _(integer)_
     - [num_tasks](#num_tasks) _(integer)_
+- [kapacitor_alert](#kapacitor_alert)
+	- [notification_dropped](#notification_dropped) _(integer)_
+	- [primary-handle-count](#primary-handle-count) _(integer)_
+	- [secondary-handle-count](#secondary-handle-count) _(integer)_
+- (Kapacitor Enterprise only) [kapacitor_cluster](#kapacitor_cluster) _(integer)_ 
+	- [dropped_member_events](#dropped_member_events) _(integer)_
+	- [dropped_user_events](#dropped_user_events) _(integer)_
+	- [query_handler_errors](#query_handler_errors) _(integer)_
 - [kapacitor_edges](#kapacitor_edges)
     - [collected](#collected) _(integer)_
     - [emitted](#emitted) _(integer)_
@@ -96,6 +104,34 @@ The total number of Kapacitor tasks.
 
 ---
 
+### kapacitor_alert
+The `kapacitor_alert` measurement stores fields with information related to
+[Kapacitor alerts](https://docs.influxdata.com/kapacitor/v1.5/working/alerts/).
+
+#### notification-dropped
+The number of internal notifications dropped because they arrive too late from another Kapacitor node. 
+If this count is increasing, Kapacitor Enterprise nodes aren't able to communicate fast enough 
+to keep up with the volume of alerts.
+
+#### primary-handle-count
+The number of times this node handled an alert as the primary. This count should increase under normal conditions.
+
+#### secondary-handle-count
+The number of times this node handled an alert as the secondary. An increase in this counter indicates that the primary is failing to handle alerts in a timely manner.
+
+---
+
+### kapacitor_cluster
+The `kapacitor_cluster` measurement reflects the ability of [Kapacitor nodes to communicate](https://docs.influxdata.com/enterprise_kapacitor/v1.5/administration/configuration/#cluster-communications) with one another. Specifically, these metrics track the gossip communication between the Kapacitor nodes.
+
+#### dropped_member_events
+The number of gossip member events that were dropped.
+
+#### dropped_user_events
+The number of gossip user events that were dropped.
+
+---
+
 ### kapacitor_edges
 The `kapacitor_edges` measurement stores fields with information related to
 [edges](https://docs.influxdata.com/kapacitor/latest/tick/introduction/#pipelines)