ci: Add GH action that clicks "enable auto-merge" on dependabot PRs (prometheus#15967)

This will merge on green all non-conflicting dependabot PRs.

Signed-off-by: bwplotka <bwplotka@gmail.com>

Author: bwplotka

.github/workflows/automerge-dependabot.yml

@@ -0,0 +1,30 @@
+---
+name: Dependabot auto-merge
+on: pull_request
+
+concurrency:
+  group: ${{ github.workflow }}-${{ (github.event.pull_request && github.event.pull_request.number) || github.ref || github.run_id }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+
+jobs:
+  dependabot:
+    permissions:
+      contents: write
+      pull-requests: write
+    runs-on: ubuntu-latest
+    if: ${{ github.event.pull_request.user.login == 'dependabot[bot]' && github.repository_owner == 'prometheus' }}
+    steps:
+    - name: Dependabot metadata
+      id: metadata
+      uses: dependabot/fetch-metadata@d7267f607e9d3fb96fc2fbe83e0af444713e90b7 # v2.3.0
+      with:
+        github-token: "${{ secrets.GITHUB_TOKEN }}"
+    - name: Enable auto-merge for Dependabot PRs
+      if: ${{steps.metadata.outputs.update-type == 'version-update:semver-minor' || steps.metadata.outputs.update-type == 'version-update:semver-patch'}}
+      run: gh pr merge --auto --merge "$PR_URL"
+      env:
+        PR_URL: ${{github.event.pull_request.html_url}}
+        GITHUB_TOKEN: ${{secrets.GITHUB_TOKEN}}