Add trivial benchmark for Rack framework to get stats for the simples…

…t possible routing

benchmarks/rack_framework/Gemfile

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+source "https://rubygems.org"
+
+gem "rack"

benchmarks/rack_framework/Gemfile.lock

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+GEM
+  remote: https://rubygems.org/
+  specs:
+    rack (3.0.8)
+
+PLATFORMS
+  arm64-darwin-21
+
+DEPENDENCIES
+  rack
+
+BUNDLED WITH
+   2.4.13

benchmarks/rack_framework/benchmark.rb

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+# frozen_string_literal: true
+
+require "harness"
+
+Dir.chdir(__dir__)
+use_gemfile
+
+require "rack"
+#require "rack/builder"
+
+# Start a server
+app = Rack::Builder.new do
+  #use Rack::CommonLogger
+  map "/ok" do
+    run lambda { |env| [200, {'content-type' => 'text/plain'}, ['OK']] }
+  end
+end
+
+#run app
+
+run_benchmark(10) do
+  10_000.times do
+    # The app may mutate `env`, so we need to create one every time.
+    env = Rack::MockRequest::env_for("http://localhost/ok")
+
+    response = app.call(env)
+    unless response[0] == 200
+      raise "HTTP response is #{response.first.inspect} instead of 200. Is the benchmark app properly set up? See README.md."
+    end
+  end
+end