Add middleware to record queue time (#34172)

spec/lib/mastodon/middleware/prometheus_queue_time_spec.rb

@@ -0,0 +1,32 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+require 'prometheus_exporter'
+require 'prometheus_exporter/middleware'
+require 'mastodon/middleware/prometheus_queue_time'
+
+RSpec.describe Mastodon::Middleware::PrometheusQueueTime do
+  subject { described_class.new(app, client:) }
+
+  let(:app) do
+    proc { |_env| [200, {}, 'OK'] }
+  end
+  let(:client) do
+    instance_double(PrometheusExporter::Client, send_json: true)
+  end
+
+  describe '#call' do
+    let(:env) do
+      {
+        'HTTP_X_REQUEST_START' => "t=#{(Time.now.to_f * 1000).to_i}",
+      }
+    end
+
+    it 'reports a queue time to the client' do
+      subject.call(env)
+
+      expect(client).to have_received(:send_json)
+        .with(hash_including(queue_time: instance_of(Float)))
+    end
+  end
+end