ci: run goldens through mypy

.github/workflows/tests.yaml

@@ -342,6 +342,15 @@ jobs:
           echo "The old one will disappear after 7 days."
       - name: Integration Tests
         run: bazel test tests/integration:asset tests/integration:credentials tests/integration:logging tests/integration:redis
+      - name: Set up Python 3.9
+        uses: actions/setup-python@v2
+        with:
+          python-version: 3.9
+      - name: Install nox.
+        run: |
+            python -m pip install nox
+      - name: Typecheck the generated output.
+        run: nox -s goldens_mypy
   style-check:
     runs-on: ubuntu-latest
     steps:

noxfile.py

@@ -294,7 +294,6 @@ def run_showcase_unit_tests(session, fail_under=100):
         *(session.posargs or [path.join("tests", "unit")]),
     )
 
-
 @nox.session(python=ALL_PYTHON)
 def showcase_unit(
     session, templates="DEFAULT", other_opts: typing.Iterable[str] = (),
@@ -370,6 +369,22 @@ def showcase_mypy_alternative_templates(session):
     showcase_mypy(session, templates=ADS_TEMPLATES, other_opts=("old-naming",))
 
 
+@nox.session(python=NEWEST_PYTHON)
+def goldens_mypy(session):
+    """Perform typecheck analysis on the golden generated libraries
+
+    This may catch additional issues that are not found in the showcase protos."""
+
+    session.install("mypy", "types-pkg-resources")
+    goldens_dir = Path("tests/integration/goldens").absolute()
+
+    for library_dir in goldens_dir.glob("*/"):
+        session.chdir(library_dir.absolute())
+        session.install(".")
+
+        session.run("mypy", "--explicit-package-bases", "google")
+
+
 @nox.session(python=NEWEST_PYTHON)
 def snippetgen(session):
     # Clone googleapis/api-common-protos which are referenced by the snippet