[chore] Add CI check to enforce merge freeze during release

.github/workflows/check-merge-freeze.yml

@@ -0,0 +1,36 @@
+name: Merge freeze
+
+on:
+  pull_request:
+    types:
+      [
+        opened,
+        ready_for_review,
+        synchronize,
+        reopened,
+        labeled,
+        unlabeled,
+        enqueued,
+      ]
+    branches: [main]
+  merge_group:
+    types: [checks_requested]
+
+permissions: read-all
+
+jobs:
+  check-merge-freeze:
+    name: Check
+    # This condition is to avoid blocking the PR causing the freeze in the first place.
+    if: |
+      (!startsWith(github.event.pull_request.title || github.event.merge_group.head_commit.message, '[chore] Prepare release')) ||
+      (!(github.event.pull_request.user.login == 'opentelemetrybot' || github.event.merge_group.head_commit.author.name == 'OpenTelemetry Bot'))
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+        with:
+          sparse-checkout: .github/workflows/scripts
+      - run: ./.github/workflows/scripts/check-merge-freeze.sh
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          REPO: open-telemetry/opentelemetry-collector-contrib

.github/workflows/scripts/check-merge-freeze.sh

@@ -0,0 +1,11 @@
+#!/bin/bash -e
+#
+# Copyright The OpenTelemetry Authors
+# SPDX-License-Identifier: Apache-2.0
+
+BLOCKERS=$( gh pr list -A opentelemetrybot -S "[chore] Prepare release" --json url -q '.[].url' -R "${REPO}" )
+if [ "${BLOCKERS}" != "" ]; then
+    echo "Merging in main is frozen, as there are open \"Prepare release\" PRs: ${BLOCKERS}"
+    echo "If you believe this is no longer true, re-run this job to unblock your PR."
+    exit 1
+fi