[tooling] Add milestone to automated backport PR (#1856)

* automatically add milestone to backport pr

* pin action by hash

Author: swang392

.github/workflows/backport-pr.yml

@@ -16,13 +16,18 @@ jobs:
     permissions:
       contents: write
       pull-requests: write
+      issues: write
+    env:
+      BRANCH_NAME: ${{ replace(github.event.label.name, 'backport/', '') }}
     steps:
-      - uses: actions/create-github-app-token@21cfef2b496dd8ef5b904c159339626a10ad380e # v1.11.6
+      - name: Create app token
+        uses: actions/create-github-app-token@21cfef2b496dd8ef5b904c159339626a10ad380e # v1.11.6
         id: app-token
         with:
           app-id: ${{ secrets.DD_GITHUBOPS_TOKEN_APP_ID }}
           private-key: ${{ secrets.DD_GITHUBOPS_TOKEN_PRIVATE_KEY }}
-      - uses: tibdex/backport@9565281eda0731b1d20c4025c43339fb0a23812e # v2.0.4
+      - name: Create backport PR
+        uses: tibdex/backport@9565281eda0731b1d20c4025c43339fb0a23812e # v2.0.4
         with:
           label_pattern: "^backport/(?<base>([^ ]+))$" 
           labels_template: "<%= JSON.stringify([...labels, 'backport', 'bot']) %>"
@@ -34,3 +39,28 @@ jobs:
             ___
 
             <%- body %>
+      
+      - name: Parse backport PR number
+        run: |
+          echo "CREATED=${{ steps.backport.outputs.created_pull_requests }}" >> $GITHUB_ENV
+          PR_NUM=$(echo "$CREATED" | jq -r ".\"${BRANCH_NAME}\"")
+          echo "PR_NUMBER=$PR_NUM" >> $GITHUB_ENV
+
+      - name: Copy milestone to backport PR
+        uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+        env:
+          PR_NUMBER: ${{ env.PR_NUMBER }}
+        with:
+          github-token: ${{ steps.app-token.outputs.token }}
+          script: |
+            const orig = context.payload.pull_request;
+            if (!orig.milestone) {
+              console.log('No milestone to copy.');
+              return;
+            }
+            await github.rest.issues.update({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              issue_number: parseInt(process.env.PR_NUMBER, 10)
+              milestone: orig.milestone.number
+            });