GH Action: Size label for PRs (#3532)

With the removal of Prow, this provides the size labeling functionality
in its stead.

Enjoy!

.github/workflows/labeler.yml

@@ -0,0 +1,36 @@
+#
+# Copyright 2023 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+name: labeler
+on:
+  - pull_request_target
+jobs:
+  labeler:
+    runs-on: ubuntu-latest
+    name: Label the PR size
+    steps:
+      - uses: codelytv/pr-size-labeler@v1
+        with:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          xs_max_size: '10'
+          s_max_size: '100'
+          m_max_size: '500'
+          l_max_size: '1000'
+          fail_if_xl: 'false'
+          message_if_xl: >
+            'This PR exceeds the recommended size of 1000 lines.
+            Please make sure you are NOT addressing multiple issues with one PR.
+            Note this PR might be rejected due to its size.’