Configure jmxfetch to use new gc metrics

This won't have any effect on jmx behavior. The next jmxfetch update
puts our desired metric names behind this feature flag. Enabling now
so we don't forget the next time we refresh jmxfetch.

dd-java-agent/agent-jmxfetch/src/main/resources/jmxfetch-config.yaml

@@ -1,5 +1,6 @@
 init_config:
   is_jmx: true
+  new_gc_metrics: true
 
 instances:
 - jmx_url: service:jmx:local:///