docs: add changelog for PR 13889

changes/ce/feat-13889.en.md

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+Improve `/api/v5/monitor_current` and `/api/v5/metrics` performance.
+
+Prior to this fix, these two APIs query clustered nodes in a loop.
+Now the queries are sent in parallel.
+The latency to return the query is dominated by the slowest node in the cluster.
+
+Also added `node` parameter to `/api/v5/monitor_current` API to avoid cluster-wide query when only need data from one node.
+For example, with `?aggregate=false&node=emqx@node1.comain.name`, the API will return data only for one node.