Allow to load the prometheus exposition module based on env vars

Useful for monitoring.

ansible/files/prosody.cfg.lua

@@ -162,7 +162,17 @@ authentication = "internal_hashed"
 authorization = "internal"
 storage = "internal"
 statistics = "internal"
-statistics_interval = 60
+
+if ENV_SNIKKET_TWEAK_PROMETHEUS == "1" then
+	-- When using Prometheus, it is desirable to let the prometheus scraping
+	-- drive the sampling of metrics
+	statistics_interval = "manual"
+else
+	-- When not using Prometheus, we need an interval so that the metrics can
+	-- be shown by the web portal. The HTTP admin API exposure does not force
+	-- a collection as it is only interested in very few specific metrics.
+	statistics_interval = 60
+end
 
 certificates = "certs"
 
@@ -200,6 +210,12 @@ VirtualHost (DOMAIN)
 		invites_register = "/register";
 	}
 
+	if ENV_SNIKKET_TWEAK_PROMETHEUS == "1" then
+		modules_enabled = {
+			"prometheus";
+		}
+	end
+
 	welcome_message = [[Hi, welcome to Snikket on $host! Thanks for joining us.]]
 	.."\n\n"
 	..[[For help and enquiries related to this service you may contact the admin via email: ]]

ansible/tasks/prosody.yml

@@ -117,6 +117,7 @@
     - mod_muc_local_only
     - mod_http_host_status_check
     - mod_measure_process
+    - mod_prometheus
 
 - name: Enable wanted modules (snikket-modules)
   file: