fix(api_core): support suppress_metrics_header fallback in AuthMetadataPlugin (#18029)

[PR #17616
](#17616)
introduced logic in grpc_helpers.py to pass
`suppress_metrics_header=True` to
`google.auth.transport.grpc.AuthMetadataPlugin`, falling back to default
args if google-auth raises a TypeError.

Following the release of `google-auth==2.56.3`, autospec=True unit test
mocks record the fallback attempt, causing
auth_metadata_plugin.assert_called_once_with to fail due to multiple
invocations.

Updates `test_create_channel_implicit_with_default_host` across sync and
async `google-api-core` unit tests to handle AuthMetadataPlugin call
assertions cleanly under both new and fallback google-auth versions.

Author: ohmayr

packages/google-api-core/tests/asyncio/test_grpc_helpers_async.py

@@ -421,9 +421,15 @@ def test_create_channel_implicit_with_default_host(
     assert channel is grpc_secure_channel.return_value
 
     google_auth_default.assert_called_once_with(scopes=None, default_scopes=None)
-    auth_metadata_plugin.assert_called_once_with(
-        mock.sentinel.credentials, mock.sentinel.Request, default_host=default_host
-    )
+    assert auth_metadata_plugin.call_count == 1
+    assert auth_metadata_plugin.call_args.args == (
+        mock.sentinel.credentials,
+        mock.sentinel.Request,
+    )
+    assert auth_metadata_plugin.call_args.kwargs["default_host"] == default_host
+    # google-auth >= 2.56.3 supports suppress_metrics_header; older versions omit it during fallback
+    if "suppress_metrics_header" in auth_metadata_plugin.call_args.kwargs:
+        assert auth_metadata_plugin.call_args.kwargs["suppress_metrics_header"] is True
     grpc_secure_channel.assert_called_once_with(
         expected_target, composite_creds, compression=None
     )

packages/google-api-core/tests/unit/test_grpc_helpers.py

@@ -453,9 +453,15 @@ def test_create_channel_implicit_with_default_host(
     assert channel is grpc_secure_channel.return_value
 
     google_auth_default.assert_called_once_with(scopes=None, default_scopes=None)
-    auth_metadata_plugin.assert_called_once_with(
-        mock.sentinel.credentials, mock.sentinel.Request, default_host=default_host
+    assert auth_metadata_plugin.call_count == 1
+    assert auth_metadata_plugin.call_args.args == (
+        mock.sentinel.credentials,
+        mock.sentinel.Request,
     )
+    assert auth_metadata_plugin.call_args.kwargs["default_host"] == default_host
+    # google-auth >= 2.56.3 supports suppress_metrics_header; older versions omit it during fallback
+    if "suppress_metrics_header" in auth_metadata_plugin.call_args.kwargs:
+        assert auth_metadata_plugin.call_args.kwargs["suppress_metrics_header"] is True
 
     grpc_secure_channel.assert_called_once_with(
         expected_target, composite_creds, compression=None