add changelog step in cicd (#4332)

This check ensures the label "skip changelog" has been applied or that the changelog has been updated. This follows the same pattern as the opentelemetry-go and opentelemetry-python SIGs.

Author: Alex Boten

.github/workflows/changelog.yml

@@ -0,0 +1,33 @@
+# This action requires that any PR targeting the main branch should touch at
+# least one CHANGELOG file. If a CHANGELOG entry is not required, add the "Skip
+# Changelog" label to disable this action.
+
+name: changelog
+
+on:
+  pull_request:
+    types: [opened, synchronize, reopened, labeled, unlabeled]
+    branches:
+      - main
+jobs:
+  changelog:
+    runs-on: ubuntu-latest
+    if: "!contains(github.event.pull_request.labels.*.name, 'Skip Changelog')"
+
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Check for CHANGELOG changes
+        run: |
+          # Only the latest commit of the feature branch is available
+          # automatically. To diff with the base branch, we need to
+          # fetch that too (and we only need its latest commit).
+          git fetch origin ${{ github.base_ref }} --depth=1
+          if [[ $(git diff --name-only FETCH_HEAD | grep CHANGELOG) ]]
+          then
+            echo "A CHANGELOG was modified. Looks good!"
+          else
+            echo "No CHANGELOG was modified."
+            echo "Please add a CHANGELOG entry, or add the \"Skip Changelog\" label if not required."
+            false
+          fi