Add Epic count to usage pings

ee/changelogs/unreleased/add-epic-count-to-usage-ping.yml

+---
+title: Add Epic count to usage pings
+merge_request:
+author:
+type: added

lib/gitlab/usage_data.rb

@@ -65,6 +65,7 @@ module Gitlab
             deploy_keys: DeployKey.count,
             deployments: Deployment.count,
             environments: ::Environment.count,
+            epics: ::Epic.count,
             geo_nodes: GeoNode.count,
             clusters: ::Clusters::Cluster.count,
             clusters_enabled: ::Clusters::Cluster.enabled.count,

spec/lib/gitlab/usage_data_spec.rb

@@ -82,6 +82,7 @@ describe Gitlab::UsageData do
         deploy_keys
         deployments
         environments
+        epics
         geo_nodes
         clusters
         clusters_enabled