Add port option to exporter middleware

lib/prometheus/middleware/exporter.rb

@@ -21,11 +21,12 @@ def initialize(app, options = {})
         @app = app
         @registry = options[:registry] || Client.registry
         @path = options[:path] || '/metrics'
+        @port = options[:port]
         @acceptable = build_dictionary(FORMATS, FALLBACK)
       end
 
       def call(env)
-        if env['PATH_INFO'] == @path
+        if metrics_port?(env['SERVER_PORT']) && env['PATH_INFO'] == @path
           format = negotiate(env, @acceptable)
           format ? respond_with(format) : not_acceptable(FORMATS)
         else
@@ -86,6 +87,10 @@ def build_dictionary(formats, fallback)
           memo[format::MEDIA_TYPE] = format
         end
       end
+
+      def metrics_port?(request_port)
+        @port.nil? || @port.to_s == request_port
+      end
     end
   end
 end

spec/prometheus/middleware/exporter_spec.rb

@@ -6,13 +6,14 @@
 describe Prometheus::Middleware::Exporter do
   include Rack::Test::Methods
 
+  let(:options) { { registry: registry } }
   let(:registry) do
     Prometheus::Client::Registry.new
   end
 
   let(:app) do
     app = ->(_) { [200, { 'Content-Type' => 'text/html' }, ['OK']] }
-    described_class.new(app, registry: registry)
+    described_class.new(app, **options)
   end
 
   context 'when requesting app endpoints' do
@@ -96,5 +97,30 @@
 
       include_examples 'ok', { 'HTTP_ACCEPT' => accept }, text
     end
+
+    context 'when a port is specified' do
+      let(:options) { { registry: registry, port: 9999 } }
+
+      context 'when a request is on the specified port' do
+        it 'responds with 200 OK' do
+          registry.counter(:foo, docstring: 'foo counter').increment(by: 9)
+
+          get 'http://example.org:9999/metrics', nil, {}
+
+          expect(last_response.status).to eql(200)
+          expect(last_response.header['Content-Type']).to eql(text::CONTENT_TYPE)
+          expect(last_response.body).to eql(text.marshal(registry))
+        end
+      end
+
+      context 'when a request is not on the specified port' do
+        it 'returns the app response' do
+          get 'http://example.org:8888/metrics', nil, {}
+
+          expect(last_response).to be_ok
+          expect(last_response.body).to eql('OK')
+        end
+      end
+    end
   end
 end