Add action to copy metadata to backport

.github/workflows/backport.yml

@@ -0,0 +1,49 @@
+name: Backport metadata
+
+# Mergify manages the opening of the backport PR, this workflow is just to extend its behaviour to
+# do useful things like copying across the tagged labels and milestone from the base PR.
+
+on:
+  pull_request_target:
+    types:
+      - opened
+    branches:
+      - 'stable/*'
+
+jobs:
+  copy_metadata:
+    name: Copy labels from base PR
+    runs-on: ubuntu-latest
+    if: github.repository == 'Qiskit/qiskit' && github.actor == 'mergify[bot]'
+
+    permissions:
+      pull-requests: write
+    env:
+      GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+    run: |
+      set -e
+      # Split Mergify's branch name (e.g. 'mergify/bp/stable/0.25/pr-10828') on the final '-' to get
+      # the number of the base PR.
+      IFS='-' read -ra split <<< "${{ github.ref_name }}"
+      base_pr=${split[-1]}
+
+      gh pr view $base_pr --json labels,milestone > base_pr.json
+
+      add_labels="$(jq '[.labels[].name] - ["stable backport potential"] | join(",")' base_pr.json)"
+      echo "New labels: '$add_labels'"
+
+      # 'jq' converts a JSON 'null' into the empty string.
+      milestone="$(jq '.milestone' base_pr.json)"
+      echo "Milestone: '$milestone'"
+
+      echo "Targetting current PR '${{ github.event.number }}'"
+      # The GitHub API is sometimes weird about calling options with empty values - the REST API
+      # certainly treats "add-label" with an empty input _not_ as a no-op but as a clear of all
+      # labels, which is quite different to its normal additive behaviour when there's labels given.
+      if [[ -n "$add_labels" && -n "$milestone" ]]; then
+        gh pr edit ${{ github.event.number }} --add-label "$add_labels" --milestone "$milestone"
+      elif [[ -n "$add_labels" ]]; then
+        gh pr edit ${{ github.event.number }} --add-label "$add_labels"
+      elif [[ -n "$milestone" ]]; then
+        gh pr edit ${{ github.event.number }} --milestone "$milestone"
+      fi