Add aggregate/union metric for Compliance tracking

Previously the app tracked unique visitors for multiple specific
features owned by the compliance group, but not the aggregate across
all these features. This introduces that aggregate so Compliance group
can better understand whether feature usage is shared or siloed.

changelogs/unreleased/292603-compliance-aggregated-usage-ping-metric.yml

+---
+title: Add aggregate/union visit tracking for Compliance features
+merge_request: 50978
+author:
+type: added

lib/gitlab/usage_data_counters/aggregated_metrics/common.yml

@@ -8,6 +8,9 @@
 #                Corresponding feature flag should have `default_enabled` attribute set to `false`.
 #                This attribute is OPTIONAL and can be omitted, when `feature_flag` is missing no feature flag will be checked.
 ---
+- name: compliance_features_track_unique_visits_union
+  operator: OR
+  events: ['g_compliance_audit_events', 'g_compliance_dashboard', 'i_compliance_audit_events', 'a_compliance_audit_events_api', 'i_compliance_credential_inventory']
 - name: product_analytics_test_metrics_union
   operator: OR
   events: ['i_search_total', 'i_search_advanced', 'i_search_paid']