docs: add changelog for PR 12714

changes/ce/fix-12714.en.md

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+Fixed some field errors in prometheus api `/prometheus/stats`.
+
+Related metrics names:
+- `emqx_cluster_sessions_count`
+- `emqx_cluster_sessions_max`
+- `emqx_cluster_nodes_running`
+- `emqx_cluster_nodes_stopped`
+- `emqx_subscriptions_shared_count`
+- `emqx_subscriptions_shared_max`
+
+Fixed the issue in endpoint: `/stats` that the values of fields `subscriptions.shared.count` and `subscriptions.shared.max`
+can not be updated in time when the client disconnected or unsubscribed the Shared-Subscription.