Improve rack middlewares

* Move from prometheus/client/rack/* to rack/middleware/*
* Use histogram instead of summary in collector middleware
* Spepicfy collector metrics by changing the prefix to http_server_*
* Remove HTTP Host header from default label builder

README.md

@@ -35,7 +35,7 @@ http_requests.increment
 ### Rack middleware
 
 There are two [Rack][2] middlewares available, one to expose a metrics HTTP
-endpoint to be scraped by a prometheus server ([Exporter][9]) and one to trace all HTTP
+endpoint to be scraped by a Prometheus server ([Exporter][9]) and one to trace all HTTP
 requests ([Collector][10]).
 
 It's highly recommended to enable gzip compression for the metrics endpoint,
@@ -45,14 +45,14 @@ for example by including the `Rack::Deflater` middleware.
 # config.ru
 
 require 'rack'
-require 'prometheus/client/rack/collector'
-require 'prometheus/client/rack/exporter'
+require 'prometheus/middleware/collector'
+require 'prometheus/middleware/exporter'
 
-use Rack::Deflater, if: ->(env, status, headers, body) { body.any? && body[0].length > 512 }
-use Prometheus::Client::Rack::Collector
-use Prometheus::Client::Rack::Exporter
+use Rack::Deflater, if: ->(_, _, _, body) { body.any? && body[0].length > 512 }
+use Prometheus::Middleware::Collector
+use Prometheus::Middleware::Exporter
 
-run ->(env) { [200, {'Content-Type' => 'text/html'}, ['OK']] }
+run ->(_) { [200, {'Content-Type' => 'text/html'}, ['OK']] }
 ```
 
 Start the server and have a look at the metrics endpoint:
@@ -179,5 +179,5 @@ rake
 [6]: https://codeclimate.com/github/prometheus/client_ruby.png
 [7]: https://coveralls.io/repos/prometheus/client_ruby/badge.png?branch=master
 [8]: https://github.com/prometheus/pushgateway
-[9]: lib/prometheus/client/rack/exporter.rb
-[10]: lib/prometheus/client/rack/collector.rb
+[9]: lib/prometheus/middleware/exporter.rb
+[10]: lib/prometheus/middleware/collector.rb

examples/rack/config.ru

@@ -1,9 +1,22 @@
 require 'rack'
-require 'prometheus/client/rack/collector'
-require 'prometheus/client/rack/exporter'
+require 'prometheus/middleware/collector'
+require 'prometheus/middleware/exporter'
 
 use Rack::Deflater, if: ->(_, _, _, body) { body.any? && body[0].length > 512 }
-use Prometheus::Client::Rack::Collector
-use Prometheus::Client::Rack::Exporter
+use Prometheus::Middleware::Collector
+use Prometheus::Middleware::Exporter
 
-run ->(_) { [200, { 'Content-Type' => 'text/html' }, ['OK']] }
+srand
+
+app = lambda do |_|
+  case rand
+  when 0..0.8
+    [200, { 'Content-Type' => 'text/html' }, ['OK']]
+  when 0.8..0.95
+    [404, { 'Content-Type' => 'text/html' }, ['Not Found']]
+  else
+    raise NoMethodError, 'It is a bug!'
+  end
+end
+
+run app

lib/prometheus/middleware/collector.rb

@@ -0,0 +1,79 @@
+# encoding: UTF-8
+
+require 'prometheus/client'
+
+module Prometheus
+  module Middleware
+    # Collector is a Rack middleware that provides a sample implementation of
+    # a HTTP tracer. The default label builder can be modified to export a
+    # different set of labels per recorded metric.
+    class Collector
+      attr_reader :app, :registry
+
+      def initialize(app, options = {}, &label_builder)
+        @app = app
+        @registry = options[:registry] || Client.registry
+        @label_builder = label_builder || DEFAULT_LABEL_BUILDER
+
+        init_request_metrics
+        init_exception_metrics
+      end
+
+      def call(env) # :nodoc:
+        trace(env) { @app.call(env) }
+      end
+
+      protected
+
+      DEFAULT_LABEL_BUILDER = proc do |env|
+        {
+          method: env['REQUEST_METHOD'].downcase,
+          path:   env['PATH_INFO'].to_s,
+        }
+      end
+
+      def init_request_metrics
+        @requests = @registry.counter(
+          :http_server_requests_total,
+          'The total number of HTTP requests handled by the Rack application.',
+        )
+        @durations = @registry.histogram(
+          :http_server_request_latency_seconds,
+          'The HTTP response latency of the Rack application.',
+        )
+      end
+
+      def init_exception_metrics
+        @exceptions = @registry.counter(
+          :http_server_exceptions_total,
+          'The total number of exceptions raised by the Rack application.',
+        )
+      end
+
+      def trace(env)
+        start = Time.now
+        yield.tap do |response|
+          duration = (Time.now - start).to_f
+          record(labels(env, response), duration)
+        end
+      rescue => exception
+        @exceptions.increment(exception: exception.class.name)
+        raise
+      end
+
+      def labels(env, response)
+        @label_builder.call(env).tap do |labels|
+          labels[:code] = response.first.to_s
+        end
+      end
+
+      def record(labels, duration)
+        @requests.increment(labels)
+        @durations.observe(labels, duration)
+      rescue
+        # TODO: log unexpected exception during request recording
+        nil
+      end
+    end
+  end
+end