Add prometheus component (#2913)

* Add prometheus component

* Update 'ha_release'

source/_components/prometheus.markdown

@@ -0,0 +1,38 @@
+---
+layout: page
+title: "Prometheus"
+description: "Record events in Prometheus."
+date: 2017-06-25 08:00
+sidebar: true
+comments: false
+sharing: true
+logo: prometheus.png
+footer: true
+ha_category: "History"
+ha_release: 0.49
+---
+
+The `prometheus` component exposes metrics in a format which [Prometheus](https://prometheus.io/) can read.
+
+To use the `prometheus` component in your installation, add the following to your `configuration.yaml` file:
+
+```yaml
+# Example configuration.yaml entry
+prometheus:
+```
+
+The Prometheus component has no configuration variables.
+
+You can then configure Prometheus to fetch metrics from Home Assistant by adding to its `scrape_configs` configuration.
+
+```yaml
+# Example Prometheus scrape_configs entry
+  - job_name: 'hass'
+    scrape_interval: 60s
+    metrics_path: /api/prometheus
+    params:
+      api_password: ['PASSWORD']
+    scheme: https
+    static_configs:
+      - targets: ['HOSTNAME:8123']
+```