spec: Rename mercure_subscribers metrics to mercure_subscribers_gauge

This change allow Datadog agent to fetch all metrics starting with mercure_subscribers
- mercure_subscribers_gauge (previously mercure_subscribers)
- mercure_subscribers_total

See #720 issue for more information

metrics.go

@@ -48,7 +48,7 @@ func NewPrometheusMetrics(registry prometheus.Registerer) *PrometheusMetrics {
 		),
 		subscribers: prometheus.NewGauge(
 			prometheus.GaugeOpts{
-				Name: "mercure_subscribers",
+				Name: "mercure_subscribers_connected",
 				Help: "The current number of running subscribers",
 			},
 		),

server_test.go

@@ -382,7 +382,7 @@ func TestMetricsCollect(t *testing.T) {
 	body = url.Values{"topic": {"http://example.com/foo/1"}, "data": {"second hello"}, "id": {"second"}}
 	server.publish(body)
 
-	server.assertMetric("mercure_subscribers 3")
+	server.assertMetric("mercure_subscribers_connected 3")
 	server.assertMetric("mercure_subscribers_total 4")
 	server.assertMetric("mercure_updates_total 2")
 }