Try GitHub actions to automatically label pull requests (#3497)

* Try GitHub actions to automatically label pull requests

* Add more labels

* Add test label

.github/labeler.yml

+document:
+  - "**/*.md"
+
+agent:
+  - "apm-sniffer/apm-sdk-plugin/**"
+  - "apm-sniffer/optional-plugins/**"
+
+plugin:
+  - "apm-sniffer/apm-sdk-plugin/**"
+  - "apm-sniffer/optional-plugins/**"
+
+core feature:
+  - "apm-sniffer/apm-agent-core/**"
+  - "oap-server/server-core/**"
+  - "oap-server/server-library/**"
+
+docker:
+  - "docker/**"
+
+OAP-backend:
+  - "oap-server/**"
+
+UI:
+  - "skywalking-ui/**"
+
+test:
+  - "test/**"
\ No newline at end of file

.github/workflows/labeler.yml

+name: Labeler
+  on: 
+    - pull_request
+  
+  jobs:
+    label:
+      runs-on: ubuntu-latest
+      steps:
+        - uses: actions/labeler@v2
+          with:
+            repo-token: "${{ secrets.GITHUB_TOKEN }}"