ci: add proto lint test with clang-format (#3228)

Summary:
We keep our protos formatted with `clang-format`, but hitherto have not
enforced this, inviting regressions. This commit adds a CI lint check.

Test Plan:
Verified that this fails on CI before the `custom_scalars/layout.proto`
fix, and passes afterward.

wchargin-branch: ci-clang-format-proto

Author: wchargin

.clang-format

@@ -0,0 +1 @@
+BasedOnStyle: google

.github/workflows/ci.yml

@@ -91,6 +91,19 @@ jobs:
         run: |
           ! git grep 'python_version = "PY2"' '*BUILD'
 
+  lint-proto:
+    runs-on: ubuntu-16.04
+    steps:
+      - uses: actions/checkout@v1
+      - name: clang-format lint
+        uses: DoozyX/[email protected]
+        with:
+          source: ./tensorboard
+          # Exclude tensorboard/compat because the source of truth is TensorFlow.
+          exclude: ./tensorboard/compat/proto
+          extensions: 'proto'
+          clangFormatVersion: 9
+
   check-misc:
     runs-on: ubuntu-16.04
     steps:

tensorboard/plugins/custom_scalar/layout.proto

@@ -17,7 +17,6 @@ syntax = "proto3";
 
 package tensorboard;
 
-
 /**
  * Encapsulates information on a single chart. Many charts appear in a category.
  */