[java-bigtable] BigTable: Invalid start_time for gauge metrics in Cloud Monitoring export

#### Environment details

1. **API**: BigTable Java Client
2. **OS type and version**: Linux (GKE environment)
3. **Java version**: 25
4. **bigtable version**: 2.66.0

#### Steps to reproduce

1. Deploy a Java application using google-cloud-bigtable 2.66.0 in GKE
2. Grant the service account `roles/monitoring.metricWriter` IAM permission
3. Application attempts to export Bigtable client metrics automatically
4. Error occurs due to invalid timestamp intervals for gauge metrics

#### Code example

```java
// Standard Bigtable client initialization
// No special configuration required to reproduce
```

#### Stack trace

```
com.google.api.gax.rpc.InvalidArgumentException: io.grpc.StatusRuntimeException: INVALID_ARGUMENT: One or more TimeSeries could not be written: Field points[0].interval.start_time had an invalid value: The start time must be equal to the end time for the gauge metric 'bigtable.googleapis.com/internal/client/grpc/subchannel/open_connections'.
at com.google.api.gax.rpc.ApiExceptionFactory.createException(ApiExceptionFactory.java:92)
at com.google.api.gax.grpc.GrpcApiExceptionFactory.create(GrpcApiExceptionFactory.java:98)
at com.google.api.gax.grpc.GrpcApiExceptionFactory.create(GrpcApiExceptionFactory.java:66)
at com.google.api.gax.grpc.GrpcExceptionCallable$ExceptionTransformingFuture.onFailure(GrpcExceptionCallable.java:97)
at com.google.api.core.ApiFutures$1.onFailure(ApiFutures.java:84)
at com.google.common.util.concurrent.Futures$CallbackListener.run(Futures.java:1125)
at com.google.common.util.concurrent.DirectExecutor.execute(DirectExecutor.java:30)
at com.google.common.util.concurrent.AbstractFuture.executeListener(AbstractFuture.java:1004)
at com.google.common.util.concurrent.AbstractFuture.complete(AbstractFuture.java:767)
at com.google.common.util.concurrent.AbstractFuture.setException(AbstractFuture.java:516)
at io.grpc.stub.ClientCalls$GrpcFuture.setException(ClientCalls.java:668)
at io.grpc.stub.ClientCalls$UnaryStreamToFuture.onClose(ClientCalls.java:638)
at io.grpc.PartialForwardingClientCallListener.onClose(PartialForwardingClientCallListener.java:39)
at io.grpc.ForwardingClientCallListener.onClose(ForwardingClientCallListener.java:23)
at io.grpc.ForwardingClientCallListener$SimpleForwardingClientCallListener.onClose(ForwardingClientCallListener.java:40)
at com.google.api.gax.grpc.ChannelPool$ReleasingClientCall$1.onClose(ChannelPool.java:569)
at io.grpc.PartialForwardingClientCallListener.onClose(PartialForwardingClientCallListener.java:39)
at io.grpc.ForwardingClientCallListener.onClose(ForwardingClientCallListener.java:23)
at io.grpc.ForwardingClientCallListener$SimpleForwardingClientCallListener.onClose(ForwardingClientCallListener.java:40)
at com.google.api.gax.grpc.GrpcLoggingInterceptor$1$1.onClose(GrpcLoggingInterceptor.java:98)
at io.grpc.internal.DelayedClientCall$DelayedListener$3.run(DelayedClientCall.java:487)
at io.grpc.internal.DelayedClientCall$DelayedListener.delayOrExecute(DelayedClientCall.java:451)
at io.grpc.internal.DelayedClientCall$DelayedListener.onClose(DelayedClientCall.java:484)
at io.grpc.internal.ClientCallImpl.closeObserver(ClientCallImpl.java:565)
at io.grpc.internal.ClientCallImpl.access$100(ClientCallImpl.java:72)
at io.grpc.internal.ClientCallImpl$ClientStreamListenerImpl$1StreamClosed.runInternal(ClientCallImpl.java:733)
at io.grpc.internal.ClientCallImpl$ClientStreamListenerImpl$1StreamClosed.runInContext(ClientCallImpl.java:714)
at io.grpc.internal.ContextRunnable.run(ContextRunnable.java:37)
at io.grpc.internal.SerializingExecutor.run(SerializingExecutor.java:133)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1090)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:614)
at java.base/java.lang.Thread.run(Thread.java:1474)
Caused by: io.grpc.StatusRuntimeException: INVALID_ARGUMENT: One or more TimeSeries could not be written
at io.grpc.Status.asRuntimeException(Status.java:532)
... 21 common frames omitted
```
#### Any additional information below

**Issue**: The Bigtable Java client (v2.66.0) incorrectly sets different `start_time` and `end_time` values for gauge metrics when exporting to Google Cloud Monitoring. According to the Cloud Monitoring API, gauge metrics must have `start_time` equal to `end_time`.

**Expected**: For gauge metrics like `bigtable.googleapis.com/internal/client/grpc/subchannel/open_connections`, start_time should equal end_time.

**Actual**: The client sets different timestamps, causing INVALID_ARGUMENT errors.

**Questions**:
1. Has this been fixed in versions newer than 2.66.0?
2. What is the recommended approach to export Bigtable metrics correctly?