modules/http: run prometheus collector on leader only

Metrics are collected and merged in http.prometheus, no need to be run
on every worker.

NEWS

@@ -13,6 +13,11 @@ Module API changes
 - kr_request::qsource.tcp renamed to ::qsource.flags.tcp
 - kr_request::has_tls renamed to ::qsource.flags.tls
 
+Bugfixes
+--------
+- http module: only run prometheus in parent process if using --forks=N,
+  as the submodule collects metrics from all sub-processes as well.
+
 
 Knot Resolver 3.1.0 (2018-11-02)
 ================================

modules/http/http.lua

@@ -366,7 +366,10 @@ end
 
 -- @function Init module
 function M.init()
-	worker.coroutine(prometheus.init)
+	-- collect and merge metrics only on leader
+	if worker.id == 0 then
+		worker.coroutine(prometheus.init)
+	end
 end
 
 -- @function Cleanup module