Add check to ensure milestone (#2747)

* Add check to ensure milestone

Stolen from https://github.com/hashicorp/vault/pull/18406

* Update .github/workflows/milestone-checker.yml

Co-authored-by: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>

* Pin grafana github actions ref

Co-authored-by: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>

.github/milestone-check.json

@@ -0,0 +1,9 @@
+[
+  {
+    "type": "check-milestone",
+    "title": "Milestone Check",
+    "success": "Milestone set",
+    "failure": "Milestone not set"
+  }
+]
+

.github/workflows/milestone-checker.yml

@@ -0,0 +1,33 @@
+# This workflow checks that there is either a 'pr/no-milestone' label applied
+# to a PR or there is a milestone associated with a PR
+
+name: Check Milestone
+
+on:
+  pull_request:
+    types: [opened, synchronize, labeled, unlabeled]
+    # Runs on PRs to main and release branches
+    branches:
+      - main
+      - release/**
+
+jobs:
+  # checks that a milestone entry is present for a PR
+  milestone-check:
+    # If there is a `pr/no-milestone` label we ignore this check
+    if: "!contains(github.event.pull_request.labels.*.name, 'pr/no-milestone')"
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout Actions
+        uses: actions/checkout@v2
+        with:
+          repository: "grafana/grafana-github-actions"
+          path: ./actions
+          ref: be89ad434792280ebaa4d982ac72ba548b6f7095
+      - name: Install Actions
+        run: npm install --production --prefix ./actions
+      - name: Run PR Checks
+        uses: ./actions/pr-checks
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+          configPath: configs/milestone-check