routes.rb

Rails.application.routes.draw do
  # ...
  get 'sidekiq-health' => SidekiqHealthMiddleware
  # ...
end

sidekiq_health_middleware.rb

class SidekiqHealthMiddleware
  LATENCY_TOLERANCE_SECONDS = ENV.fetch('SIDEKIQ_LATENCY_TOLERANCE', 5).to_f

  def body
    healthy? ? 'OK' : 'UHOH'
  end

  def healthy?
    Sidekiq::Queue.new.latency < LATENCY_TOLERANCE_SECONDS
  end

  def headers
    { 'Content-Type' => 'text/plain' }
  end

  def status_code
    200
  end

  def response
    [status_code, headers, [body]]
  end

  def call(*)
    response
  end

  def self.call(*params)
    new.call(*params)
  end
end

sidekiq_health_middleware_spec.rb

describe SidekiqHealthMiddleware do
  describe '.call' do
    subject { described_class.call }
    let(:queue) { double :queue }
    let(:headers) { { 'Content-Type' => 'text/plain' } }
    let(:response) { [200, headers, [body]] }

    before do
      allow(Sidekiq::Queue).to receive(:new) { queue }
      allow(queue).to receive(:latency) { latency }
    end

    context 'ok' do
      let(:latency) { described_class::LATENCY_TOLERANCE_SECONDS - 1 }
      it { is_expected.to match [200, headers, ['OK']] }
    end

    context 'not ok' do
      let(:latency) { described_class::LATENCY_TOLERANCE_SECONDS + 1 }
      it { is_expected.to match [200, headers, ['UHOH']] }
    end
  end
end