Add Prometheus-Documentation (#556)

* Add Prometheus-Documentation

* Prometheus-Documentation: Fix indentation

* format

Co-authored-by: Guillermo Ruffino <glm.net@gmail.com>

components/web_server.rst

@@ -30,13 +30,14 @@ interface are hosted by esphome.io. If you want to use your own service, use the
     web_server:
       port: 80
 
+
 Configuration variables:
 ------------------------
 
 - **port** (*Optional*, int): The port the web server should open its socket on.
 - **css_url** (*Optional*, url): The URL that should be used for the CSS stylesheet. Defaults
   to https://esphome.io/_static/webserver-v1.min.css (updates will go to ``v2``, ``v3``, etc). Can be set to empty string.
-- **css_include** (*Optional*, local file): Path to local file to be included in web server index page. 
+- **css_include** (*Optional*, local file): Path to local file to be included in web server index page.
   Contents of this file will be served as ``/0.css`` and used as CSS stylesheet by internal webserver.
   Useful when building device without internet access, where you want to use built-in AP and webserver.
 - **js_url** (*Optional*, url): The URL that should be used for the JS script. Defaults
@@ -49,6 +50,7 @@ Configuration variables:
   - **username** (**Required**, string): The username to use for authentication.
   - **password** (**Required**, string): The password to check for authentication.
 
+- **prometheus** (*Optional*, boolean): Enable the prometheus-endpoint. See :ref:`prometheus-exporter`.
 - **id** (*Optional*, :ref:`config-id`): Manually specify the ID used for code generation.
 
 .. note::
@@ -63,6 +65,7 @@ Configuration variables:
           auth:
             username: admin
             password: !secret web_server_password
+          prometheus: true
 
     Example web_server configuration with CSS and JS included from esphome-docs.
     CSS and JS URL's are set to empty value, so no internet access is needed for this device to show it's web interface.
@@ -77,6 +80,34 @@ Configuration variables:
           js_include: "../../../esphome-docs/_static/webserver-v1.min.js"
           js_url: ""
 
+
+.. _prometheus-exporter:
+
+Prometheus Exporter:
+--------------------
+
+The webserver can provide an `Prometheus <https://prometheus.io/>`__-Exporter under ``/metrics``.
+
+This can be used to scrape data directly into your Prometheus-based monitoring and alerting-system,
+without the need of any other software.
+
+The prometheus-endpoint is activated by setting ``prometheus: true`` in the ``web_server``-area of
+the configuration.
+
+The list of available metrics can be found by directly browsing your node under
+``<ip or node_name.local>/metrics``, and may be increased in the future.
+
+.. note::
+
+    Example integration into the configuration of your prometheus:
+
+    .. code-block:: yaml
+
+        scrape_configs:
+          - job_name: esphome
+            static_configs:
+              - targets: [<ip or node_name.local>]
+
 See Also
 --------