Add admin docs about monitoring.

docs/admin/04_monitoring.rst

+Monitoring
+##########
+
+Prometheus
+**********
+
+AlekSIS provides a metric endpoint at `/metrics`, so you can scrape metrics in
+your Prometheus instance.
+
+Available metrics
+=================
+
+The exporter provides metrics about responses and requests, e.g.  statistics
+about response codes, request latency and requests per view.  It also
+provides data about database operations.
+
+Prometheus config to get metrics
+================================
+
+To get metrics of your AlekSIS instance, just add the following to your
+`prometheus.yml`::
+
+  - job_name: aleksis
+    static_configs:
+      - targets: ['my.aleksis-instance.com']
+    metrics_path: /metrics
+
+
+Grafana
+*******
+
+Visualise metrics with Grafana
+==============================
+
+If you want to visualise your AlekSIS metrics with Grafana, you can use one
+of the public available Grafana dashboards, for example the following one,
+or just write your own.
+
+https://grafana.com/grafana/dashboards/9528