Avoid iterating all prepared statements when getting PreparedStatementsCacheSize metric

Patch by marcuse; reviewed by Dmitry Konstantinov for CASSANDRA-21038

Author: krummas

CHANGES.txt

@@ -1,4 +1,5 @@
 5.1
+ * Avoid iterating all prepared statements when getting PreparedStatementsCacheSize metric (CASSANDRA-21038)
  * Reduce performance impact of TableMetadataRef.get and KeyspaceMetadataRef.get (CASSANDRA-20465)
  * Improve CMS initialization (CASSANDRA-21036)
  * Introducing comments and security labels for schema elements (CASSANDRA-20943)

src/java/org/apache/cassandra/cql3/QueryProcessor.java

@@ -181,12 +181,9 @@ public static int preparedStatementsCount()
 
     public static long preparedStatementsCacheMemoryUsedBytes()
     {
-        long preparedStatementsCacheMemoryUsedBytes = 0;
-        for (Map.Entry<MD5Digest, Prepared> entry : preparedStatements.asMap().entrySet())
-        {
-            preparedStatementsCacheMemoryUsedBytes += getSizeOfPreparedStatementForCache(entry.getKey(), entry.getValue());
-        }
-        return preparedStatementsCacheMemoryUsedBytes;
+        return preparedStatements.policy().eviction()
+                                 .map(p -> p.weightedSize().orElse(0L))
+                                 .orElse(0L);
     }
 
     // Work around initialization dependency