Update DCM triggers to match Dart DevTools (#2230)

Author: elliette

.github/workflows/dcm.yml

@@ -4,7 +4,9 @@ name: Dart Code Metrics
 
 on:
   pull_request_target:
-    types: [opened, reopened, labeled, unlabeled]
+    # labeled and unlabeled are required along with open to re-run
+    # workflow when the run-dcm-workflow label is added or removed.
+    types: [opened, labeled, unlabeled]
     branches:
       - master