ensure can report mau stats when hs.config.mau_stats_only is set (#4305)

* ensure can report mau stats when hs.config.mau_stats_only is set

changelog.d/4305.bugfix

@@ -0,0 +1 @@
+The metric synapse_admin_mau:current previously did not update when config.mau_stats_only was set to True

synapse/app/homeserver.py

@@ -537,7 +537,7 @@ def run(hs):
         )
 
     start_generate_monthly_active_users()
-    if hs.config.limit_usage_by_mau:
+    if hs.config.limit_usage_by_mau or hs.config.mau_stats_only:
         clock.looping_call(start_generate_monthly_active_users, 5 * 60 * 1000)
     # End of monthly active user settings