Add docs for Prometheus integration

docs/Grafana.md

@@ -1,5 +1,14 @@
 ![logo](_media/logo/grafana-logo.png)
 
-@samvdb has published a Grafana dashboard at https://grafana.com/grafana/dashboards/15940-ems-esp
+This page features all EMS-ESP Community Grafana dashboards:
+
+| Dashboard   | Author                               | Database & Datasource   |
+|-------------|--------------------------------------|------------|
+| [EMS-ESP](https://grafana.com/grafana/dashboards/15940-ems-esp)     | [@samvdb](https://github.com/samvdb) | InfluxDB / Telegraf   |
+
+
+<br> 
+
+# Screenshots
 
 ![dashboard](_media/screenshot/grafana-dashboard.png)

docs/Integrations.md

@@ -10,6 +10,7 @@ EMS-ESP integrates with a few home automation systems to report status and senso
 [![IOBroker](_media/logo/iobroker-logo.png)](IOBroker.md)
 [![Domoticz](_media/logo/domoticz.png)](Domoticz.md)
 [![openHAB](_media/logo/openhab-logo.png)](openHAB.md)
+[![Prometheus](_media/logo/prometheus-logo.png)](Prometheus.md)
 [![Grafana](_media/logo/grafana-logo.png)](Grafana.md)
 [![loxone](_media/logo/loxone-logo.png)](Loxone.md)
 [![modbus](_media/logo/modbus.jpg)](Modbus.md)

docs/Prometheus.md

@@ -0,0 +1,29 @@
+![logo](_media/logo/prometheus-logo.png)
+
+EMS-ESP exposes the `/api/<device>/metrics` endpoint for integration with Prometheus.
+At this point, only numeric and boolean (as 0 & 1) values are returned by the api.
+All EMS-ESP metrics are prefixed with `emsesp_`.
+
+The following sample configuration can be used as `prometheus.yml`:
+```YML
+global:
+  scrape_interval: 15s
+
+scrape_configs:
+  - job_name: 'emsesp'
+    static_configs:
+      - targets: ['ems-esp.local']
+        labels:
+          device: boiler
+      - targets: ['ems-esp.local']
+        labels:
+          device: thermostat
+#     - some other device ...
+
+    relabel_configs:
+      - source_labels: [device]
+        target_label: __metrics_path__
+        regex: (.*)
+        replacement: /api/$1/metrics
+```
+To visualize the Prometheus metrics, see also: [Integrations: Grafana](Grafana.md)