HBASE-27494: Fix missing meta cache dropping exception metrics (#4902)

Signed-off-by: Bryan Beaudreault <bbeaudreault@apache.org>

Author: briaugenreich

hbase-client/src/main/java/org/apache/hadoop/hbase/client/AsyncRequestFutureImpl.java

@@ -912,6 +912,10 @@ private void cleanServerCache(ServerName server, Throwable regionException) {
     if (ClientExceptionsUtil.isMetaClearingException(regionException)) {
       // We want to make sure to clear the cache in case there were location-related exceptions.
       // We don't to clear the cache for every possible exception that comes through, however.
+      MetricsConnection metrics = asyncProcess.connection.getConnectionMetrics();
+      if (metrics != null) {
+        metrics.incrCacheDroppingExceptions(regionException);
+      }
       asyncProcess.connection.clearCaches(server);
     }
   }