Revert "v8.5: remove VictoriaMetrics guide" (#21678)

Author: hfxsd

maintain-tidb-using-tiup.md

@@ -283,7 +283,7 @@ The destroy operation stops the services and clears the data directory and deplo
 tiup cluster destroy ${cluster-name}
 ```
 
-<!-- ## Switch from Prometheus to VictoriaMetrics
+## Switch from Prometheus to VictoriaMetrics
 
 In large-scale clusters, Prometheus might encounter performance bottlenecks when handling a large number of instances. Starting from TiUP v1.16.3, TiUP supports switching the monitoring component from Prometheus to VictoriaMetrics (VM) to provide better scalability, higher performance, and lower resource consumption.
 
@@ -433,4 +433,3 @@ After confirming that the old metrics have expired, you can perform the followin
     ```bash
     rm -rf /tidb-data/prometheus-8249
     ```
--->