const flowControlOptions = {
    flowControl: { maxMessages: 10 }
  };

    const subscription = await pubsubClient.subscription(
    '/projects/my-topic/my-subscription',
    flowControlOptions
  );

(node:6) UnhandledPromiseRejectionWarning: Error: 14 UNAVAILABLE: No connection established
    at Object.callErrorFromStatus (/app/node_modules/@google-cloud/pubsub/node_modules/@grpc/grpc-js/build/src/call.js:30:26)
    at Object.onReceiveStatus (/app/node_modules/@google-cloud/pubsub/node_modules/@grpc/grpc-js/build/src/client.js:390:49)
    at Object.onReceiveStatus (/app/node_modules/@google-cloud/pubsub/node_modules/@grpc/grpc-js/build/src/client-interceptors.js:304:181)
    at Http2CallStream.outputStatus (/app/node_modules/@google-cloud/pubsub/node_modules/@grpc/grpc-js/build/src/call-stream.js:116:74)
    at Http2CallStream.maybeOutputStatus (/app/node_modules/@google-cloud/pubsub/node_modules/@grpc/grpc-js/build/src/call-stream.js:155:22)
    at Http2CallStream.endCall (/app/node_modules/@google-cloud/pubsub/node_modules/@grpc/grpc-js/build/src/call-stream.js:141:18)
    at Http2CallStream.cancelWithStatus (/app/node_modules/@google-cloud/pubsub/node_modules/@grpc/grpc-js/build/src/call-stream.js:457:14)
    at ChannelImplementation.tryPick (/app/node_modules/@google-cloud/pubsub/node_modules/@grpc/grpc-js/build/src/channel.js:237:32)
    at ChannelImplementation._startCallStream (/app/node_modules/@google-cloud/pubsub/node_modules/@grpc/grpc-js/build/src/channel.js:267:14)
    at Http2CallStream.start (/app/node_modules/@google-cloud/pubsub/node_modules/@grpc/grpc-js/build/src/call-stream.js:435:22)
(node:6) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1)