docs(alerting): add info about alerting metrics

docs/sources/alerting/metrics.md

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+title = "Alerting Metrics"
+description = "Alerting Metrics Guide"
+keywords = ["Grafana", "alerting", "guide", "metrics"]
+type = "docs"
+[menu.docs]
+name = "Metrics"
+parent = "alerting"
+weight = 2
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+
+# Metrics from the alert engine
+
+> Alerting is only available in Grafana v4.0 and above.
+
+The alert engine publish some internal metrics about itself. You can read more about how Grafana published [interal metrics](/installation/configuration/#metrics)
+
+Description | Type | Metric name
+---------- | ----------- | ----------
+Total number of alerts | counter | `alerting.active_alerts`
+Alert execution result | counter | `alerting.result`
+Notifications sent counter | counter | `alerting.notifications_sent`
+Alert execution timer | timer | `alerting.execution_time`