Simple hardware metrics collection + alerting (#115)

* Add CPU and RAM usage alerting

* Create basic troubleshooting document to point alerts at

* Limit max number of hardware values collected

* Save metric value with the point in time it was taken

metrics/alerting.go

@@ -0,0 +1,41 @@
+package metrics
+
+import (
+	log "github.com/sirupsen/logrus"
+)
+
+const maxCPUAlertingThresholdPCT = 95
+const maxRAMAlertingThresholdPCT = 95
+
+const alertingError = "The %s utilization of %d%% is higher than the alerting threshold of %d%%.  This can cause issues with video generation and delivery. Please visit the documentation at https://github.com/gabek/owncast/blob/master/doc/troubleshooting.md to help troubleshoot this issue."
+
+func handleAlerting() {
+	handleCPUAlerting()
+	handleRAMAlerting()
+}
+
+func handleCPUAlerting() {
+	if len(Metrics.CPUUtilizations) < 2 {
+		return
+	}
+
+	avg := recentAverage(Metrics.CPUUtilizations)
+	if avg > maxCPUAlertingThresholdPCT {
+		log.Errorf(alertingError, "CPU", avg, maxCPUAlertingThresholdPCT)
+	}
+}
+
+func handleRAMAlerting() {
+	if len(Metrics.RAMUtilizations) < 2 {
+		return
+	}
+
+	avg := recentAverage(Metrics.RAMUtilizations)
+	if avg > maxRAMAlertingThresholdPCT {
+		log.Errorf(alertingError, "memory", avg, maxRAMAlertingThresholdPCT)
+	}
+}
+
+func recentAverage(values []value) int {
+	return int((values[len(values)-1].Value + values[len(values)-2].Value) / 2)
+}