doc: fix metrics example

Signed-off-by: Yong Wang <wang.yong19@zte.com.cn>
Acked-by: Remy Horton <remy.horton@intel.com>

doc/guides/prog_guide/metrics_lib.rst

@@ -144,6 +144,7 @@ print out all metrics for a given port:
 .. code-block:: c
 
     void print_metrics() {
+        struct rte_metric_value *metrics;
         struct rte_metric_name *names;
         int len;