HPCC-27443 Fix potential crash in metric manager

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>

system/jlib/jmetrics.hpp

@@ -51,6 +51,8 @@ enum MetricType
  */
 interface IMetric
 {
+    virtual ~IMetric() = default;
+
     /*
      * Returns the metric name
      */