Try to fix Errno::EADDRINUSE in metrics_spec

* in the GitHub unit test action, we often see multiple errors "Address already in use - bind(2) for "127.0.0.1" port 9395"
* try to use an ephemeral port for each test

Author: jochenehret

spec/request/internal/metrics_spec.rb

@@ -3,19 +3,28 @@
 RSpec.describe 'Metrics' do
   let(:user) { VCAP::CloudController::User.make }
   let(:user_header) { headers_for(user) }
-  let(:metrics_webserver) { VCAP::CloudController::MetricsWebserver.new }
 
-  delegate :app, to: :metrics_webserver
+  around(:example) do |group|
+    RSpec::Mocks.with_temporary_scope do
+      # Force Puma to bind to an ephemeral port (0) to avoid EADDRINUSE
+      allow_any_instance_of(Puma::Server).to receive(:add_tcp_listener).and_wrap_original do |m, host, _|
+        m.call(host, 0)
+      end
 
-  before do
-    allow_any_instance_of(VCAP::CloudController::Metrics::PeriodicUpdater).to receive(:update_webserver_stats)
-    metrics_webserver.start(TestConfig.config_instance)
-  end
+      @metrics_webserver = VCAP::CloudController::MetricsWebserver.new
+      allow_any_instance_of(VCAP::CloudController::Metrics::PeriodicUpdater).to receive(:update_webserver_stats)
+      @metrics_webserver.start(TestConfig.config_instance)
 
-  after do
-    metrics_webserver.stop
+      begin
+        group.run
+      ensure
+        @metrics_webserver&.stop
+      end
+    end
   end
 
+  delegate :app, to: :@metrics_webserver
+
   it 'can be called several times' do
     get '/internal/v4/metrics', nil