Documentation for Usage Ping aggregated metrics

doc/development/product_analytics/usage_ping.md

@@ -681,6 +681,50 @@ with any of the other services that are running. That is not how node metrics ar
 always runs as a process alongside other GitLab components on any given node. From Usage Ping's perspective none of the node data would therefore
 appear to be associated to any of the services running, since they all appear to be running on different hosts. To alleviate this problem, the `node_exporter` in GCK was arbitrarily "assigned" to the `web` service, meaning only for this service `node_*` metrics will appear in Usage Ping.
 
+## Aggregated metrics
+
+> - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/45979) in GitLab 13.6.
+> - It's [deployed behind a feature flag](../../user/feature_flags.md), disabled by default.
+> - It's enabled on GitLab.com.
+
+CAUTION: **Warning:**
+This feature is intended solely for internal GitLab use.
+
+In order to add data for aggregated metrics into Usage Ping payload you should add corresponding definition into  [`aggregated_metrics.yml`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/usage_data_counters/aggregated_metrics.yml) file. Each aggregate definition includes following parts:
+
+- name: unique name under which aggregate metric will be added to Usage Ping payload
+- operator: operator that defines how aggregated metric data will be counted. Available operators are:
+  - `ANY`: removes duplicates and counts all entries that triggered any of listed events
+- events: list of events names (from [`known_events.yml`](#known-events-in-usage-data-payload)) to aggregate into metric. All events in this list must have the same `redis_slot` and `aggregation` attributes.
+
+Example aggregated metric entry:
+
+```yaml
+- name: example_aggregated_metric
+  operator: ANY
+  events: ['i_search_total', 'i_search_advanced', 'i_search_paid']
+```
+
+Aggregated metrics will be added under `aggregated_metrics` key in both `counts_weekly` and `counts_monthly` top level keys in Usage Ping payload.
+
+```ruby
+{
+  :counts_monthly => {
+    :deployments => 1003,
+    :successful_deployments => 78,
+    :failed_deployments => 275,
+    :packages => 155,
+    :personal_snippets => 2106,
+    :project_snippets => 407,
+    :promoted_issues => 719,
+    :aggregated_metrics => {
+      :example_aggregated_metric => 7
+    },
+    :snippets => 2513
+  }
+}
+```
+
 ## Example Usage Ping payload
 
 The following is example content of the Usage Ping payload.