Add block vs ensure benchmark

.gitignore

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+/vendor/bundle
+/.envrc

block-ensure/README.md

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+```
+                   user     system      total        real
+block          0.961390   0.008933   0.970323 (  0.978850)
+ensure         0.752044   0.006407   0.758451 (  0.763439)
+```

block-ensure/example.rb

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+#!/usr/bin/env ruby
+
+require 'benchmark'
+
+class Example
+  def initialize
+    @mutex = Mutex.new
+  end
+
+  def using_block
+    @mutex.synchronize do
+
+    end
+  end
+
+  def using_ensure
+    @mutex.lock
+  ensure
+    @mutex.unlock
+  end
+end
+
+Benchmark.bm(12) do |x|
+  ex = Example.new
+  n = 10_000_000
+
+  x.report('block') { n.times { ex.using_block } }
+  x.report('ensure') { n.times { ex.using_ensure } }
+end

shell.nix

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+{
+  # use the environment channel
+  pkgs ? import <nixpkgs> {},
+
+  # use a pinned package state
+  pinned ? import(fetchTarball("https://github.com/NixOS/nixpkgs/archive/14d9b465c71.tar.gz")) {},
+}:
+let
+  # specify ruby version to use
+  ruby = pinned.ruby_3_1;
+
+  # control llvm/clang version (e.g for packages built from source)
+  llvm = pinned.llvmPackages_12;
+
+  # control gcc version (e.g for packages built from source)
+  gcc = pinned.gcc12;
+in llvm.stdenv.mkDerivation {
+  # unique project name for this environment derivation
+  name = "dd-trace-rb.devshell";
+
+  buildInputs = [
+    ruby
+
+    # TODO: some gems insist on using `gcc` on Linux, satisfy them for now:
+    # - json
+    # - protobuf
+    # - ruby-prof
+    gcc
+  ];
+
+  shellHook = ''
+    # get major.minor.0 ruby version
+    export RUBY_VERSION="$(ruby -e 'puts RUBY_VERSION.gsub(/\d+$/, "0")')"
+
+    # make gem install work in-project, compatibly with bundler
+    export GEM_HOME="$(pwd)/vendor/bundle/ruby/$RUBY_VERSION"
+
+    # make bundle work in-project
+    export BUNDLE_PATH="$(pwd)/vendor/bundle"
+
+    # enable calling gem scripts without bundle exec
+    export PATH="$GEM_HOME/bin:$PATH"
+  '';
+}