Customers can verify call options

[dbolduc]

Our APIs look like:

T Client::Foo(FooRequest const& request, Options options);
T Connection::Foo(FooReqeust const& request);

Customers can verify they are giving the right request to our library. They cannot verify that they are giving the right options to our library.

void MakeClientCall() {
  // This function can be unit tested by customers
  auto options = MakeCallOptions();

  // But they can never catch the difference between:
  client.Foo(req, options);
  // and...
  client.Foo(req); // oops I forgot to include the options.
}

I think the solution is to add:

namespace google::cloud::mocks {
/**
 * Verify per-call options
 * 
 * @code
 * TEST(Foo, CallOptions) {
 *   auto mock = std::make_shared<MockConnection>();
 *   EXPECT_CALL(*mock, Foo).WillOnce([] {
 *         auto options = google::cloud::mocks::GetCallOptions();
 *         EXPECT_THAT(options, ...);
 *       });
 *   auto client = Client(mock);
 *   DoThing(client);
 * }
 * @endcode
 */
Options GetCallOptions() {
  return internal::CurrentOptions();
}
}