Add alerts to apache mixin

prometheus/apache-http-mixin/alerts/alerts.libsonnet

@@ -0,0 +1,52 @@
+{
+  prometheusAlerts+:: {
+    groups+: [
+      {
+        name: 'apache-http-alerts',
+        rules: [
+          {
+            alert: 'ApacheDown',
+            expr: 'apache_up == 0',
+            labels: {
+              severity: 'warning',
+            },
+            annotations: {
+              summary: 'Apache is down.',
+              description: 'Apache is down on {{ $labels.instance }}.',
+            },
+            'for': '5m',
+          },
+          {
+            alert: 'ApacheRestart',
+            expr: 'apache_uptime_seconds_total / 60 < 1',
+            labels: {
+              severity: 'info',
+            },
+            annotations: {
+              summary: 'Apache restart.',
+              description: 'Apache has just been restarted.',
+            },
+            'for': '0',
+          },
+          {
+            alert: 'ApacheWorkersLoad',
+            expr: |||
+              (sum by (instance) (apache_workers{state="busy"}) / sum by (instance) (apache_scoreboard) ) * 100 > %(alertsWarningWorkersBusy)s
+            ||| % $._config,
+            'for': '15m',
+            labels: {
+              severity: 'warning',
+            },
+            annotations: {
+              summary: 'Apache workers load is too high.',
+              description: |||
+                Apache workers in busy state approach the max workers count %(alertsWarningWorkersBusy)s%% workers busy on {{ $labels.instance }}.
+                The currect value is {{ $value }}%%.
+              ||| % $._config,
+            },
+          },
+        ],
+      },
+    ],
+  },
+}

prometheus/apache-http-mixin/config.libsonnet

@@ -4,5 +4,8 @@
     dashboardPeriod: 'now-1h',
     dashboardTimezone: 'default',
     dashboardRefresh: '1m',
+
+    // for alerts
+    alertsWarningWorkersBusy: '80', // %
   },
 }

prometheus/apache-http-mixin/mixin.libsonnet

@@ -1,3 +1,3 @@
 (import 'dashboards/apache-exporter-full.libsonnet') +
-// (import 'alerts/alerts.libsonnet') +
+(import 'alerts/alerts.libsonnet') +
 (import 'config.libsonnet')