Add failing tests for integrating in rack

Signed-off-by: Ahmad Tolba <tolpa1@gmail.com>

spec/prometheus/integration_spec.rb

@@ -0,0 +1,31 @@
+# encoding: UTF-8
+
+require 'rack/test'
+require 'rack'
+require 'prometheus/middleware/collector'
+require 'prometheus/middleware/exporter'
+
+API = Rack::Builder.new do
+  use Rack::Deflater
+  use Prometheus::Middleware::Collector
+  use Prometheus::Middleware::Exporter
+
+  map "/" do
+    run ->(_) { [200, {'Content-Type' => 'text/html'}, ['OK']] }
+  end
+end
+
+describe API do
+  include Rack::Test::Methods
+
+  let(:app) { described_class }
+
+  context 'GET /metrics' do
+    it 'fails on the second request' do
+      get '/metrics'
+      expect { last_response }.not_to raise_error
+      expect { get '/metrics' }.not_to raise_error
+    end
+  end
+end
+