Fix master lost file count metric is always 0

Cherry-pick of existing commit.
orig-pr: #14401
orig-commit: 5c2f77e
orig-commit-author: Bowen Ding <bowen@alluxio.com>

pr-link: #14415
change-id: cid-eeb1e6c6cbd9821c7f07f9bc6eb58863550e68b7

core/common/src/main/java/alluxio/metrics/MetricKey.java

@@ -348,16 +348,17 @@ public MetricKey build() {
               + "Use this metric to monitor whether your journal is running out of disk space.")
           .setMetricType(MetricType.GAUGE)
           .build();
-  public static final MetricKey MASTER_LOST_FILE_COUNT =
-      new Builder("Master.LostFileCount")
-          .setDescription("Count of lost files")
-          .setMetricType(MetricType.GAUGE)
-          .build();
   public static final MetricKey MASTER_LOST_BLOCK_COUNT =
       new Builder("Master.LostBlockCount")
           .setDescription("Count of lost unique blocks")
           .setMetricType(MetricType.GAUGE)
           .build();
+  public static final MetricKey MASTER_LOST_FILE_COUNT =
+      new Builder("Master.LostFileCount")
+          .setDescription("Count of lost files. This number is cached and may not be in sync with "
+              + String.format("%s", MetricKey.MASTER_LOST_BLOCK_COUNT.getName()))
+          .setMetricType(MetricType.GAUGE)
+          .build();
   public static final MetricKey MASTER_TOTAL_PATHS =
       new Builder("Master.TotalPaths")
           .setDescription("Total number of files and directory in Alluxio namespace")

core/server/master/src/main/java/alluxio/master/file/LostFileDetector.java

@@ -45,7 +45,7 @@ public LostFileDetector(FileSystemMaster fileSystemMaster, InodeTree inodeTree)
     mFileSystemMaster = fileSystemMaster;
     mInodeTree = inodeTree;
     MetricsSystem.registerCachedGaugeIfAbsent(MetricKey.MASTER_LOST_FILE_COUNT.getName(),
-        mFileSystemMaster.getLostFiles()::size);
+        () -> mFileSystemMaster.getLostFiles().size());
   }
 
   @Override