Add Prometheus for some Owncast metrics (#1744)

* Add Prometheus for some Owncast metrics. Closes #1303

* Wrap prometheus metrics endpoint in admin middleware

metrics/prometheus.go

@@ -0,0 +1,14 @@
+package metrics
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+var (
+	labels                  map[string]string
+	activeViewerCount       prometheus.Gauge
+	activeChatClientCount   prometheus.Gauge
+	cpuUsage                prometheus.Gauge
+	chatUserCount           prometheus.Gauge
+	currentChatMessageCount prometheus.Gauge
+)