Update example README for #111

examples/rack/README.md

@@ -44,21 +44,20 @@ Prometheus server can be used to [play around with the metrics][rate-query].
 The example shown in [`config.ru`](config.ru) is a trivial rack application
 using the default collector and exporter middlewares.
 
-In order to use custom label builders in the collector, change the line to
-something like this:
-
-```ruby
-use Prometheus::Middleware::Collector, counter_label_builder: ->(env, code) {
-  next { code: nil, method: nil, host: nil, path: nil } if env.empty?
-
-  {
-    code:         code,
-    method:       env['REQUEST_METHOD'].downcase,
-    # Include the HTTP Host header as label.
-    host:         env['HTTP_HOST'].to_s,
-    # Include path, but replace all numeric IDs to keep cardinality low.
-    # Think '/users/1234/comments' -> '/users/:id/comments'
-    path:         env['PATH_INFO'].to_s.gsub(/\/\d+(\/|$)/, '/:id\\1'),
-  }
-}
+Modifying the labels is a subject under development (see #111) but one option is to subclass `Prometheus::Middleware::Collector` and override the methods you need. For example, if you want to [strip IDs from the path](https://github.com/prometheus/client_ruby/blob/982fe2e3c37e2940d281573c7689224152dd791f/lib/prometheus/middleware/collector.rb#L97-L101) you could override the appropriate method:
+
+```Ruby
+require 'prometheus/middleware/collector'
+module Prometheus
+  module Middleware
+    class MyCollector < Collector
+      def strip_ids_from_path(path)
+        super(path)
+          .gsub(/8675309/, ':jenny\\1')
+      end
+    end
+  end
+end
 ```
+
+and in `config.ru` use your class instead.