Add a flag to make per-host metrics optional

When serving many hosts from one nginx, the metrics may become too
numerous for Prometheus.  Add a flag to disable the host label, so
that metrics are totalled across all hosts.

cmd/nginx/flags.go

@@ -149,6 +149,8 @@ Feature backed by OpenResty Lua libraries. Requires that OCSP stapling is not en
 
 		enableMetrics = flags.Bool("enable-metrics", true,
 			`Enables the collection of NGINX metrics`)
+		metricsPerHost = flags.Bool("metrics-per-host", true,
+			`Export metrics per-host`)
 
 		httpPort      = flags.Int("http-port", 80, `Port to use for servicing HTTP traffic.`)
 		httpsPort     = flags.Int("https-port", 443, `Port to use for servicing HTTPS traffic.`)
@@ -227,6 +229,7 @@ Feature backed by OpenResty Lua libraries. Requires that OCSP stapling is not en
 		ElectionID:                 *electionID,
 		EnableProfiling:            *profiling,
 		EnableMetrics:              *enableMetrics,
+		MetricsPerHost:             *metricsPerHost,
 		EnableSSLPassthrough:       *enableSSLPassthrough,
 		EnableSSLChainCompletion:   *enableSSLChainCompletion,
 		ResyncPeriod:               *resyncPeriod,

cmd/nginx/main.go

@@ -131,7 +131,7 @@ func main() {
 
 	mc := metric.NewDummyCollector()
 	if conf.EnableMetrics {
-		mc, err = metric.NewCollector(conf.ListenPorts.Status, reg)
+		mc, err = metric.NewCollector(conf.ListenPorts.Status, conf.MetricsPerHost, reg)
 		if err != nil {
 			klog.Fatalf("Error creating prometheus collector:  %v", err)
 		}