add release gem spec

spec/datadog/ci/release_gem_spec.rb

@@ -0,0 +1,59 @@
+RSpec.describe "gem release process" do
+  context "datadog-ci.gemspec" do
+    context "files" do
+      subject(:files) { Gem::Specification.load("datadog-ci.gemspec").files }
+
+      # It's easy to forget to ship new files, especially when a new paradigm is
+      # introduced (e.g. introducing native files requires the inclusion `ext/`)
+      it "includes all important files" do
+        single_files_excluded = /
+          ^
+          (
+           |\.env
+           |\.gitignore
+           |\.rspec
+           |\.rubocop.yml
+           |\.standard.yml
+           |\.standard_todo.yml
+           |\.simplecov
+           |\.yardopts
+           |Appraisals
+           |CODE_OF_CONDUCT.md
+           |CONTRIBUTING.md
+           |Gemfile
+           |Gemfile-.*
+           |Rakefile
+           |Steepfile
+           |datadog-ci\.gemspec
+           |docker-compose\.yml
+          )
+          $
+        /x
+
+        directories_excluded = %r{
+          ^(
+            spec
+            |docs
+            |\.circleci
+            |\.github
+            |\.vscode
+            | bin
+            |gemfiles
+            |integration
+            |tasks
+            |yard
+            |vendor/rbs
+          )/
+        }x
+
+        expect(files)
+          .to match_array(
+            `git ls-files -z`
+              .split("\x0")
+              .reject { |f| f.match(directories_excluded) }
+              .reject { |f| f.match(single_files_excluded) }
+          )
+      end
+    end
+  end
+end