HDFS-16202. Use constants "HdfsClientConfigKeys.Failover.PREFIX" instead of "dfs.client.failover."

Author: weixiaoxing

hadoop-hdfs-project/hadoop-hdfs-client/src/main/java/org/apache/hadoop/hdfs/server/namenode/ha/ObserverReadProxyProvider.java

@@ -84,7 +84,7 @@ public class ObserverReadProxyProvider<T>
 
   /** Configuration key for {@link #observerProbeRetryPeriodMs}. */
   static final String OBSERVER_PROBE_RETRY_PERIOD_KEY =
-      "dfs.client.failover.observer.probe.retry.period";
+      HdfsClientConfigKeys.Failover.PREFIX + "observer.probe.retry.period";
   /** Observer probe retry period default to 10 min. */
   static final long OBSERVER_PROBE_RETRY_PERIOD_DEFAULT = 60 * 10 * 1000;