add test coverage

src/grpc.ts

@@ -291,9 +291,15 @@ export class GrpcClient {
     return this._getCredentials(options).then(credentials => {
       const grpcOptions: {[index: string]: {}} = {};
       Object.keys(options).forEach(key => {
-        if (key.indexOf('grpc.') === 0 || key.indexOf('grpcgcp.') === 0) {
+        if (key.indexOf('grpc.') === 0){
           grpcOptions[key] = options[key];
         }
+        if (key.indexOf('grpc_gcp.') === 0) {
+          // This prefix is used to pass additional arguments that aren't
+          // options for grpc. Strip the prefix before passing.
+          const prefixLength = 'grpc_gcp.'.length;
+          grpcOptions[key.substr(prefixLength)] = options[key];
+        }
       });
       return new CreateStub(serviceAddress, credentials, grpcOptions);
     });

test/grpc.ts

@@ -180,6 +180,7 @@ describe('grpc', () => {
         port: 443,
         'grpc.max_send_message_length': 10 * 1024 * 1024,
         'grpc.initial_reconnect_backoff_ms': 10000,
+        'grpc_gcp.non_grpc_option': 1,
         other_dummy_options: 'test',
       };
       return grpcClient.createStub(DummyStub, opts).then(stub => {
@@ -188,8 +189,9 @@ describe('grpc', () => {
         expect(stub.creds).to.deep.eq(dummyChannelCreds);
         // tslint:disable-next-line no-any
         (expect(stub.options).has as any).key([
-          'max_send_message_length',
-          'initial_reconnect_backoff_ms',
+          'grpc.max_send_message_length',
+          'grpc.initial_reconnect_backoff_ms',
+          'non_grpc_option'
         ]);
         // tslint:disable-next-line no-any
         (expect(stub.options).to.not.have as any).key([