Update link to upstream Kafka docs

the new consumer is now the standard consumer, so they dropped the `new_` from the anchor

Author: jeffwidman

kafka/consumer/group.py

@@ -918,7 +918,7 @@ def metrics(self, raw=False):
         """Get metrics on consumer performance.
 
         This is ported from the Java Consumer, for details see:
-        https://kafka.apache.org/documentation/#new_consumer_monitoring
+        https://kafka.apache.org/documentation/#consumer_monitoring
 
         Warning:
             This is an unstable interface. It may change in future