Doc for usage data batch counts by a grouped relation

doc/development/telemetry/usage_ping.md

@@ -223,6 +223,25 @@ Examples:
 sum(JiraImportState.finished, :imported_issues_count)
 ```
 
+### Grouping & Batch Operations
+
+The `count`, `distinct_count`, and `sum` batch counters can accept an `ActiveRecord::Relation`
+object, which groups by a specified column. With a grouped relation, the methods do batch counting, 
+handle errors, and returns a hash table of key-value pairs.
+
+Examples:
+
+```ruby
+count(Namespace.group(:type))
+# returns => {nil=>179, "Group"=>54}
+
+distinct_count(Project.group(:visibility_level), :creator_id)
+# returns => {0=>1, 10=>1, 20=>11}
+
+sum(Issue.group(:state_id), :weight))
+# returns => {1=>3542, 2=>6820}
+```
+
 ### Redis Counters
 
 Handles `::Redis::CommandError` and `Gitlab::UsageDataCounters::BaseCounter::UnknownEvent`