Provide a sample application using Spanner emulator

Hi,

I'm getting the following exception after running a simple integration test. I've tried various combinations and I always hit the same error. I'm not sure if it is a configuration issue on my end or a bug since the information during the debugging was not sufficient to come up to a conclusion on what exactly the problem is.
It would be super beneficial if there was a sample app using Spanner + Spring Boot + R2DBC + Testcontainers (Spanner Emulator). The same way we have it for Postgres for example.

```
java.lang.AssertionError: expectation "expectComplete" failed (expected: onComplete(); actual: onError(com.google.cloud.spanner.SpannerException: UNAUTHENTICATED: com.google.api.gax.rpc.UnauthenticatedException: io.grpc.StatusRuntimeException: UNAUTHENTICATED: Request is missing required authentication credential. Expected OAuth 2 access token, login cookie or other valid authentication credential. See https://developers.google.com/identity/sign-in/web/devconsole-project.))

at reactor.test.MessageFormatter.assertionError(MessageFormatter.java:115)
at reactor.test.MessageFormatter.failPrefix(MessageFormatter.java:104)
at reactor.test.MessageFormatter.fail(MessageFormatter.java:73)
at reactor.test.MessageFormatter.failOptional(MessageFormatter.java:88)
at reactor.test.DefaultStepVerifierBuilder.lambda$expectComplete$4(DefaultStepVerifierBuilder.java:336)
at reactor.test.DefaultStepVerifierBuilder$SignalEvent.test(DefaultStepVerifierBuilder.java:2218)
at reactor.test.DefaultStepVerifierBuilder$DefaultVerifySubscriber.onSignal(DefaultStepVerifierBuilder.java:1490)
at reactor.test.DefaultStepVerifierBuilder$DefaultVerifySubscriber.onExpectation(DefaultStepVerifierBuilder.java:1438)
at reactor.test.DefaultStepVerifierBuilder$DefaultVerifySubscriber.onError(DefaultStepVerifierBuilder.java:1091)
at reactor.core.publisher.FluxFlatMap$FlatMapMain.checkTerminated(FluxFlatMap.java:842)
at reactor.core.publisher.FluxFlatMap$FlatMapMain.drainLoop(FluxFlatMap.java:608)
at reactor.core.publisher.FluxFlatMap$FlatMapMain.drain(FluxFlatMap.java:588)
at reactor.core.publisher.FluxFlatMap$FlatMapMain.onError(FluxFlatMap.java:451)
at reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onError(FluxOnErrorResume.java:106)
at reactor.core.publisher.Operators.error(Operators.java:198)
at reactor.core.publisher.FluxError.subscribe(FluxError.java:43)
at reactor.core.publisher.Flux.subscribe(Flux.java:8469)
at reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onError(FluxOnErrorResume.java:103)
at reactor.core.publisher.FluxUsingWhen$UsingWhenSubscriber.deferredError(FluxUsingWhen.java:398)
at reactor.core.publisher.FluxUsingWhen$RollbackInner.onComplete(FluxUsingWhen.java:475)
at reactor.core.publisher.Operators$MultiSubscriptionSubscriber.onComplete(Operators.java:2058)
at reactor.core.publisher.MonoCreate$DefaultMonoSink.success(MonoCreate.java:133)
at reactor.core.publisher.MonoCreate$DefaultMonoSink.success(MonoCreate.java:144)
at com.google.cloud.spanner.r2dbc.v2.DatabaseClientReactiveAdapter$1.onSuccess(DatabaseClientReactiveAdapter.java:320)
at com.google.api.core.ApiFutures$1.onSuccess(ApiFutures.java:73)
at com.google.common.util.concurrent.Futures$CallbackListener.run(Futures.java:1139)
at reactor.core.scheduler.SchedulerTask.call(SchedulerTask.java:68)
at reactor.core.scheduler.SchedulerTask.call(SchedulerTask.java:28)
at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
at java.base/java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:304)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
at java.base/java.lang.Thread.run(Thread.java:833)
Suppressed: com.google.cloud.spanner.SpannerException: UNAUTHENTICATED: com.google.api.gax.rpc.UnauthenticatedException: io.grpc.StatusRuntimeException: UNAUTHENTICATED: Request is missing required authentication credential. Expected OAuth 2 access token, login cookie or other valid authentication credential. See https://developers.google.com/identity/sign-in/web/devconsole-project.
at com.google.cloud.spanner.SpannerExceptionFactory.newSpannerExceptionPreformatted(SpannerExceptionFactory.java:284)
at com.google.cloud.spanner.SpannerExceptionFactory.newSpannerException(SpannerExceptionFactory.java:170)
at com.google.cloud.spanner.SpannerExceptionFactory.newSpannerException(SpannerExceptionFactory.java:110)
at com.google.cloud.spanner.SessionPool$WaiterFuture.pollUninterruptiblyWithTimeout(SessionPool.java:1558)
at com.google.cloud.spanner.SessionPool$WaiterFuture.get(SessionPool.java:1529)
at com.google.cloud.spanner.SessionPool$WaiterFuture.get(SessionPool.java:1506)
at com.google.common.util.concurrent.ForwardingFuture.get(ForwardingFuture.java:66)
at com.google.cloud.spanner.SessionPool$PooledSessionFuture.get(SessionPool.java:1307)
at com.google.cloud.spanner.SessionPool$AutoClosingReadContext$1.internalNext(SessionPool.java:289)
at com.google.cloud.spanner.SessionPool$AutoClosingReadContext$1.next(SessionPool.java:252)
at com.google.cloud.spanner.AsyncResultSetImpl$ProduceRowsCallable.call(AsyncResultSetImpl.java:340)
at com.google.cloud.spanner.AsyncResultSetImpl$ProduceRowsCallable.call(AsyncResultSetImpl.java:334)
at com.google.common.util.concurrent.TrustedListenableFutureTask$TrustedFutureInterruptibleTask.runInterruptibly(TrustedListenableFutureTask.java:131)
at com.google.common.util.concurrent.InterruptibleTask.run(InterruptibleTask.java:74)
at com.google.common.util.concurrent.TrustedListenableFutureTask.run(TrustedListenableFutureTask.java:82)
at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:539)
... 5 more
Caused by: com.google.cloud.spanner.SpannerException: UNAUTHENTICATED: com.google.api.gax.rpc.UnauthenticatedException: io.grpc.StatusRuntimeException: UNAUTHENTICATED: Request is missing required authentication credential. Expected OAuth 2 access token, login cookie or other valid authentication credential. See https://developers.google.com/identity/sign-in/web/devconsole-project.
at com.google.cloud.spanner.SpannerExceptionFactory.newSpannerExceptionPreformatted(SpannerExceptionFactory.java:284)
at com.google.cloud.spanner.SpannerExceptionFactory.newSpannerException(SpannerExceptionFactory.java:170)
at com.google.cloud.spanner.SpannerExceptionFactory.newSpannerException(SpannerExceptionFactory.java:110)
at com.google.cloud.spanner.SessionPool$SessionConsumerImpl.onSessionCreateFailure(SessionPool.java:2315)
at com.google.cloud.spanner.SessionClient$BatchCreateSessionsRunnable.run(SessionClient.java:140)
... 6 more
Caused by: com.google.cloud.spanner.SpannerException: UNAUTHENTICATED: com.google.api.gax.rpc.UnauthenticatedException: io.grpc.StatusRuntimeException: UNAUTHENTICATED: Request is missing required authentication credential. Expected OAuth 2 access token, login cookie or other valid authentication credential. See https://developers.google.com/identity/sign-in/web/devconsole-project.
at com.google.cloud.spanner.SpannerExceptionFactory.newSpannerExceptionPreformatted(SpannerExceptionFactory.java:284)
at com.google.cloud.spanner.SpannerExceptionFactory.newSpannerException(SpannerExceptionFactory.java:61)
at com.google.cloud.spanner.SpannerExceptionFactory.newSpannerException(SpannerExceptionFactory.java:181)
at com.google.cloud.spanner.spi.v1.GapicSpannerRpc.get(GapicSpannerRpc.java:1676)
at com.google.cloud.spanner.spi.v1.GapicSpannerRpc.batchCreateSessions(GapicSpannerRpc.java:1399)
at com.google.cloud.spanner.SessionClient.internalBatchCreateSessions(SessionClient.java:299)
at com.google.cloud.spanner.SessionClient.access$000(SessionClient.java:38)
at com.google.cloud.spanner.SessionClient$BatchCreateSessionsRunnable.run(SessionClient.java:137)
... 6 more
Caused by: java.util.concurrent.ExecutionException: com.google.api.gax.rpc.UnauthenticatedException: io.grpc.StatusRuntimeException: UNAUTHENTICATED: Request is missing required authentication credential. Expected OAuth 2 access token, login cookie or other valid authentication credential. See https://developers.google.com/identity/sign-in/web/devconsole-project.
at com.google.common.util.concurrent.AbstractFuture.getDoneValue(AbstractFuture.java:588)
at com.google.common.util.concurrent.AbstractFuture.get(AbstractFuture.java:567)
at com.google.cloud.spanner.spi.v1.GapicSpannerRpc.get(GapicSpannerRpc.java:1670)
... 10 more
Caused by: com.google.api.gax.rpc.UnauthenticatedException: io.grpc.StatusRuntimeException: UNAUTHENTICATED: Request is missing required authentication credential. Expected OAuth 2 access token, login cookie or other valid authentication credential. See https://developers.google.com/identity/sign-in/web/devconsole-project.
at com.google.api.gax.rpc.ApiExceptionFactory.createException(ApiExceptionFactory.java:71)
at com.google.api.gax.grpc.GrpcApiExceptionFactory.create(GrpcApiExceptionFactory.java:72)
at com.google.api.gax.grpc.GrpcApiExceptionFactory.create(GrpcApiExceptionFactory.java:60)
at com.google.api.gax.grpc.GrpcExceptionCallable$ExceptionTransformingFuture.onFailure(GrpcExceptionCallable.java:97)
at com.google.api.core.ApiFutures$1.onFailure(ApiFutures.java:68)
at com.google.common.util.concurrent.Futures$CallbackListener.run(Futures.java:1133)
at com.google.common.util.concurrent.DirectExecutor.execute(DirectExecutor.java:31)
at com.google.common.util.concurrent.AbstractFuture.executeListener(AbstractFuture.java:1277)
at com.google.common.util.concurrent.AbstractFuture.complete(AbstractFuture.java:1038)
at com.google.common.util.concurrent.AbstractFuture.setException(AbstractFuture.java:808)
at io.grpc.stub.ClientCalls$GrpcFuture.setException(ClientCalls.java:564)
at io.grpc.stub.ClientCalls$UnaryStreamToFuture.onClose(ClientCalls.java:534)
at io.grpc.PartialForwardingClientCallListener.onClose(PartialForwardingClientCallListener.java:39)
at io.grpc.ForwardingClientCallListener.onClose(ForwardingClientCallListener.java:23)
at io.grpc.ForwardingClientCallListener$SimpleForwardingClientCallListener.onClose(ForwardingClientCallListener.java:40)
at io.grpc.PartialForwardingClientCallListener.onClose(PartialForwardingClientCallListener.java:39)
at io.grpc.ForwardingClientCallListener.onClose(ForwardingClientCallListener.java:23)
at io.grpc.ForwardingClientCallListener$SimpleForwardingClientCallListener.onClose(ForwardingClientCallListener.java:40)
at com.google.cloud.spanner.spi.v1.SpannerErrorInterceptor$1$1.onClose(SpannerErrorInterceptor.java:100)
at io.grpc.internal.DelayedClientCall$DelayedListener$3.run(DelayedClientCall.java:463)
at io.grpc.internal.DelayedClientCall$DelayedListener.delayOrExecute(DelayedClientCall.java:427)
at io.grpc.internal.DelayedClientCall$DelayedListener.onClose(DelayedClientCall.java:460)
at io.grpc.internal.ClientCallImpl.closeObserver(ClientCallImpl.java:562)
at io.grpc.internal.ClientCallImpl.access$300(ClientCallImpl.java:70)
at io.grpc.internal.ClientCallImpl$ClientStreamListenerImpl$1StreamClosed.runInternal(ClientCallImpl.java:743)
at io.grpc.internal.ClientCallImpl$ClientStreamListenerImpl$1StreamClosed.runInContext(ClientCallImpl.java:722)
at io.grpc.internal.ContextRunnable.run(ContextRunnable.java:37)
at io.grpc.internal.SerializingExecutor.run(SerializingExecutor.java:133)
... 6 more
Caused by: io.grpc.StatusRuntimeException: UNAUTHENTICATED: Request is missing required authentication credential. Expected OAuth 2 access token, login cookie or other valid authentication credential. See https://developers.google.com/identity/sign-in/web/devconsole-project.
at io.grpc.Status.asRuntimeException(Status.java:535)
... 23 more
```

I've created this simple minimalistic app with just the dependencies mentioned above to narrow down the perimeter:
https://github.com/magiccrafter/spanner-spring-boot-r2jdbc-app

I've followed the Credentials section in the README without luck.
https://github.com/GoogleCloudPlatform/cloud-spanner-r2dbc#authentication

> In the absence of explicit authentication options, Application Default Credentials will be automatically inferred from the environment in which the application is running, unless the connection is in plain-text, indicating the use of Cloud Spanner emulator.