Update dendrite.yaml.j2

roles/custom/matrix-dendrite/templates/dendrite/dendrite.yaml.j2

@@ -416,3 +416,6 @@ tracing:
 # stdout by Dendrite.
 logging: []
 
+# statistics reporting configuration. These statistics contain the server
+# name, number of active users and some information on your deployment config.
+report_stats: {{ matrix_dendrite_report_stats|to_json }}