Fix the cache unclear bug. (#4739)

* Fix the cache unclear bug.

* Update LimitedSizeBufferedData.java
Co-authored-by: Nkezhenxu94 <kezhenxu94@apache.org>

oap-server/server-core/src/main/java/org/apache/skywalking/oap/server/core/analysis/data/LimitedSizeBufferedData.java

@@ -74,8 +74,12 @@ public class LimitedSizeBufferedData<STORAGE_DATA extends ComparableStorageData
 
     @Override
     public List<STORAGE_DATA> read() {
-        List<STORAGE_DATA> collection = new ArrayList<>();
-        data.values().forEach(e -> e.forEach(collection::add));
-        return collection;
+        try {
+            List<STORAGE_DATA> collection = new ArrayList<>();
+            data.values().forEach(collection::addAll);
+            return collection;
+        } finally {
+            data.clear();
+        }
     }
 }

oap-server/server-core/src/main/java/org/apache/skywalking/oap/server/core/analysis/data/MergableBufferedData.java

@@ -57,6 +57,10 @@ public class MergableBufferedData<METRICS extends Metrics> implements BufferedDa
 
     @Override
     public List<METRICS> read() {
-        return buffer.values().stream().collect(Collectors.toList());
+        try {
+            return buffer.values().stream().collect(Collectors.toList());
+        } finally {
+            buffer.clear();
+        }
     }
 }