testing: explicitly 0-initialize CassMetrics struct

Without this, valgrind complains about access to uninitialized memory.

Author: muzarski

tests/src/integration/objects/session.hpp

@@ -111,7 +111,7 @@ class Session : public Object<CassSession, cass_session_free> {
    * @return Driver metrics
    */
   CassMetrics metrics() const {
-    CassMetrics metrics;
+    CassMetrics metrics = {};
     cass_session_get_metrics(get(), &metrics);
     return metrics;
   }