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.