Add Github workflow for Dart Code Metrics (#2092)

Author: elliette

.github/workflows/dcm.yml

@@ -0,0 +1,35 @@
+# A CI workflow to run the Dart Code Metrics analyzer (https://dcm.dev).
+
+name: Dart Code Metrics
+
+on:
+  pull_request_target
+
+jobs:
+  dcm:
+    name: Dart Code Metrics
+    runs-on: ubuntu-latest
+    steps:
+      - name: Install DCM
+        run: |
+          sudo apt-get update
+          wget -qO- https://dcm.dev/pgp-key.public | sudo gpg --dearmor -o /usr/share/keyrings/dcm.gpg
+          echo 'deb [signed-by=/usr/share/keyrings/dcm.gpg arch=amd64] https://dcm.dev/debian stable main' | sudo tee /etc/apt/sources.list.d/dart_stable.list
+          sudo apt-get update
+          sudo apt-get install dcm
+          sudo chmod +x /usr/bin/dcm
+      - name: Setup Dart SDK
+        uses: dart-lang/setup-dart@d6a63dab3335f427404425de0fbfed4686d93c4f
+        with:
+          sdk: dev
+      - id: checkout
+        name: Checkout repository
+        uses: actions/checkout@8f4b7f84864484a7bf31766abe9204da3cbe65b3
+      - id: dwds_pub_upgrade
+        name: dwds; dart pub upgrade
+        run: dart pub upgrade
+        if: "always() && steps.checkout.conclusion == 'success'"
+        working-directory: dwds
+      - name: Run DCM on DWDS
+        run: dcm analyze --ci-key="${{ secrets.DCM_CI_KEY }}" --email="${{ secrets.DCM_EMAIL }}" lib
+        working-directory: dwds