Add changelog label to merged PRs (#3664)

.github/workflows/add_changelog_label.yaml

@@ -0,0 +1,26 @@
+name: Add changelog label to merged PR
+on:
+  pull_request:
+    types: [closed]
+
+jobs:
+  build:
+    if: github.event.pull_request.merged
+    runs-on: ubuntu-latest
+    steps:
+      - name: Apply label
+        uses: actions/github-script@0.4.0
+        with:
+          github-token: ${{secrets.GITHUB_TOKEN}}
+          script: |
+            const is_changelog_label = (label) => label.name.startsWith('Changelog Entry: ');
+            if (context.payload.pull_request.labels.some(is_changelog_label)) {
+                console.log('Issue already has Changelog Entry label, skipping...');
+                return;
+            }
+            github.issues.addLabels({
+              issue_number: context.issue.number,
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              labels: ['Changelog Entry: Pending']
+            });