Add example for custom label builders in collector middleware

Fixes #29.

examples/rack/README.md

@@ -3,10 +3,31 @@
 A simple Rack application which shows how to use prometheus' `Rack::Exporter`
 and `Rack::Collector` rack middlwares.
 
-## Usage
+## Run the example
 
 Execute the provided `run` script:
 
 ```bash
 ./run
 ```
+
+This will start the rack app, run a few requests against it and print the
+output of `/metrics`.
+
+## Overview
+
+The example show in [`config.ru`](config.ru) is a trivial rack application
+using the available collector and exporter middlewares.
+
+In order to use a custom label builder, change the line to something like this:
+
+```ruby
+use Prometheus::Client::Rack::Collector do |env|
+  {
+    method:     env['REQUEST_METHOD'].downcase,
+    host:       env['HTTP_HOST'].to_s,
+    path:       env['PATH_INFO'].to_s,
+    user_agent: env['HTTP_USER_AGENT'].to_s,
+  }
+end
+```