Danger: Add bundle size analysis

Adding a new danger job which analyses and compares the current MRs
bundle size to the bundle size of the commit on master it is trying to
merge into.

The tool itself is managed in:
https://gitlab.com/gitlab-org/frontend/playground/webpack-memory-metrics

.gitlab/ci/frontend.gitlab-ci.yml

@@ -335,3 +335,22 @@ webpack-dev-server:
     expire_in: 31d
     paths:
       - webpack-dev-server.json
+
+bundle-size-review:
+  extends:
+    - .default-retry
+    - .frontend:rules:bundle-size-review
+  image: registry.gitlab.com/gitlab-org/gitlab-build-images:danger
+  stage: test
+  needs: ["gitlab:assets:compile pull-cache"]
+  script:
+    - mkdir -p bundle-size-review
+    - cp webpack-report/index.html bundle-size-review/bundle-report.html
+    - yarn global add https://gitlab.com/gitlab-org/frontend/playground/webpack-memory-metrics.git
+    - danger --dangerfile=danger/bundle_size/Dangerfile --fail-on-errors=true --verbose --danger_id=bundle-size-review
+  artifacts:
+    when: always
+    name: bundle-size-review
+    expire_in: 31d
+    paths:
+      - bundle-size-review

.gitlab/ci/rules.gitlab-ci.yml

@@ -335,6 +335,12 @@
       changes: *frontend-dependency-patterns
       allow_failure: true
 
+.frontend:rules:bundle-size-review:
+  rules:
+    - if: '$DANGER_GITLAB_API_TOKEN && $CI_MERGE_REQUEST_IID && $CI_MERGE_REQUEST_TARGET_BRANCH_NAME == "master"'
+      changes: *frontend-patterns
+      allow_failure: true
+
 ################
 # Memory rules #
 ################

danger/bundle_size/Dangerfile

+# frozen_string_literal: true
+
+analysis_result = "./bundle-size-review/analysis.json"
+markdown_result = "./bundle-size-review/comparison.md"
+
+# Executing the webpack-entry-point-analyser
+# We would like to do that in the CI file directly,
+# but unfortunately the head_commit SHA is not available
+# as a CI variable due to our merge into master simulation
+analyze_cmd = [
+  "webpack-entry-point-analyser",
+  "--from-file ./webpack-report/stats.json",
+  "--json #{analysis_result}",
+  " --sha #{gitlab&.head_commit}"
+].join(" ")
+
+# execute analysis
+`#{analyze_cmd}`
+
+# We are executing the comparison by comparing the start_sha
+# to the current pipeline result. The start_sha is the commit
+# from master that was merged into for the merged pipeline.
+comparison_cmd = [
+  "webpack-compare-reports",
+  "--from-sha #{gitlab.mr_json["diff_refs"]["start_sha"]}",
+  "--to-file #{analysis_result}",
+  "--html ./bundle-size-review/comparison.html",
+  "--markdown #{markdown_result}"
+].join(" ")
+
+# execute comparison
+`#{comparison_cmd}`
+
+comment = `cat #{markdown_result}`
+
+markdown(<<~MARKDOWN)
+    #{comment}
+MARKDOWN